On days like these, kids like you... should be dancing in Hell! (What I thought when I heard that Techno battle theme)

Hmm, looked a bit too easy but then again I'm of the opinion that a boss is too easy unless it kills someone.

Since it has a snake tail, it could also use an Acid Spit attack at your face. Can Blind and/or Poison you.

As this is a guaranteed encounter, you could also have it accompanied by some minions to make it unique compared to the random encounter version. Or not? Maybe it'd be too overwhelming to have it able to Roar and call for allies.

Can the Mass-Fire attack not inflict Burn? Then again, maybe that'd be overkill having to spend some actions to cure everyone.

Hmm, attacks that trigger when you're statused would be good to have. They come in many flavors. Weak Kill in Devil Survivor dealt more damage to status-ed targets, Death's Door reduces the HP of all sick characters to 1, and Death Call instantly kills all charmed/paralyzed/or muted characters. That's kind of incentive to cure yourself, don't you think? Though there's also the classic 'Doomed' status which instantly kills the afflicted when the duration ends.

I'm really fond of auto-life ever since I fought a dracolich boss. Just when I thought I had killed it, it revives. It's a nasty surprise. Then there's a two-headed Zombie Hydra I fought. It's gimmick is that its heads each have their own HP bar and action. But you can't kill one head and make the battle easier on you because the other head will full-revive it in a few turns. So to beat it, you need to reduce them both to low HP and then kill them both in the same turn.

A monster's death doesn't mean you're safe. I also love "If I'm gonna die, I'm taking you with me/doing as much damage as I can before that happens." It's pretty cool when characters in fiction are heavily outnumbered, wounded, and tired but manage to slaughter a lot of enemies before finally being taken down. Basically, a killed monster will deal one final shot at you when it dies. It might stab you when you don't expect it, it might use its last breath to buff or heal its allies or debuff you, and it might even do something like explode in your face.

Heck, you can even have a monster that activates a Determination Last Stand state when you deal a lethal hit to them. The way it works is that they'll have 1 HP no matter what you do to them until the duration of the status ends at which point they'll instantly die. So the only way to stop them without incurring damage is to immobilize them with a status.

Oh right, I forgot to ask: do you have a way to decrease maximum HP temporarily equal to the damage dealt? In Fate Stay Night, there is a spear called Gae Buidhe which does exactly this. A character can normally heal themselves indefinitely but what happens when their max HP is reduced? Well, it means they have to end the battle quickly before their max HP is reduced to a threshold where they'll almost always get killed by an attack even with dmg reduction buffs/debuffs in place. It also means you would have to have a second Taunt/Tank to switch to when the first one is weakened too much.

Of course, as I mentioned before in a previous post, a status that halves how much you heal or nullifies it altogether is also an effective tactic for stopping players from healing themselves indefinitely. It basically makes them really have to rely on defensive buffs/attack reduction skills because they can't so easily heal the damage off.

Oh, and I remember fighting a mini-boss called Bog Lurker in Etrian Odyssey. It's actually really easy but its skill set does essentially wall off brute-force. First it pumps up its defense, and then it uses life drain which damages you and heals it. Simple but very effective if you haven't invested in any defense debuff skills by then. I suppose you could consider it a minor 'Wake-up Call Boss' where you can't mash attack until it dies (well, without first dropping its defenses anyway).

I suppose you can't really do this since your characters have like single-digit MP but attacks that damage both your HP and MP are a pretty effective player-killer as will. It means you definitely need items or a way to regenerate MP when a monster can bite off the last remaining MP you had that you were planning to use. Well, I suppose you can still make attacks that take off 1 or 2 MP.

Skill ideas:

Strength Sapper: Boss skill. Deals normal damage but also decreases Atk. of target and buffs own Atk. but only if it hits.

Clear Mind: Boss skill. Buffs own M.Atk/M.Def while dispelling Clatter from enemy party.

Bloodlust: Normal attack that heals self for a percentage of the damage dealt. Increased damage and accuracy against bleeding target.

Adaptability: Boss passive. When struck by an element attack, gains resistance to that element after taking damage. If struck by a different element afterwards, modifiers reset and resistance is applied to the new element.

Retribution: Boss skill. Normal attack whose damage is multiplied by the number of times the user has been hit before using it.

Element Shift: Boss passive. Changes elemental weakness/resist every few turns. Also changes skill set of spells to the resisted element (a fire-resistant enemy will gain fire spells).

Zombify: Target gains Undead status. Gains weakness to Heal/Light-element attacks.

Poison Discharge AKA You poisoned the wrong monster: Used only by poisoned plant-type enemies like mushrooms. Cures self and inflicts enemy party with poison via noxious fumes.

Sludge: Enemy skill. Magic damage. Has a chance of inflicting Poison and/or Blind. 100% chance to inflict Oil.

Fall from Grace: Boss skill. Magic damage. Deals more damage to a target based on the number of buffs they have.

Boss Ideas: A boss with multiple bodies. If you kill one body, the rest gain immunity/resist against the element you used (including physical) to do the deed. An advanced form of the adaptability passive; Countermeasure.

After fighting a zombie enemy in a game, a life-drain enemy really does seem even more appealing. Basically, I tried to nuke it and managed to take off off 66% of its health with a charged light spell. But then it life drained me. Since I didn't reduce its Atk, buff my Def, and had no resistance to the darkness element, it healed all that damage back. Of course, that only happened because life drains for enemies in this game are essentially double the damage dealt. >___>

I suppose a tough enemy could be made like that. Buff Atk, Buff Def, Nerf your Def, Buff Evasion, spell that damages the party and gives them a bleed DoT, and then Bloodlust to drain you. If you don't use buffs/debuffs, you won't be able to out-damage its healing. Though I suppose people make find it time-consuming or annoying to fight such a monster.

Then there's the Battle Alchemist. Nice chap who sells you stuff. Potions can have beneficial or nasty effects. The latter type you'll find being thrown at you if you try to rob him (option only available if you accept a side-quest from a guy to kill off his competition. You get discounted prices at his shop as a reward).

Oil Spray: Douses your whole team with Oil.
Liquid Fire: Now that you're weak to fire, you'll all take some heavy fire damage and start burning.

Corrosive Acid: Deals two hits; physical (the glass vial breaking) which can cause you to bleed and non-elemental magic damage (the acid itself). The second hit's accuracy depends on whether the first one hit or not. Melts through the target's armor, reducing their defenses.

Dirty Bomb: Explodes randomly in 2-4 turns, dealing non-elemental magic damage to the whole party. Be prepared for when that happens. It's basically a delayed attack. The fact that the damage is "saved" for the next turn(s) makes it dangerous. Radioactive waste has high chance to poison and/or blind.

Exhaustion Gas: Decreases Atk, MAtk, and Agility of target. Also removes 2 MP. Yeah, this guy might be way too OP.

Blitzkrieg Potion: User drinks it to increase Atk, MAtk, and Agility.
Ironskin Potion: User drinks it to increase Def. and MDef.
Invisibility Potion: User drinks it to increase Evasion, Accuracy, and Critical Hit chance.
Life Potion: User drinks it to not only restore HP but also gain the Regeneration status.
Panacea: User drinks it to cure all statuses.
Interrupt: The player can knock the above potions out of the Alchemist's hand with a fast attack such as Quick Stab.

Obtainable Items:
Holy Water: A potion to throw at your enemies. Deals light-element damage to undead and demons. May burn and/or debuff them. Deals water-element damage to everything else.

Pesticide: A potion to throw at your enemies. Deals fire-element damage to plants and insects. May poison and/or debuff them. Deals water-element damage to everything else but still keeps the poison chance.

Monster Potion: Transforms you into a monster with special skills and possible resist/weakness to certain types of attacks or statuses.

Mana Bomb: Throw at your enemy to deal non-elemental magic damage. Deals more damage against high MAtk target (Especially if MAtk is buffed).

Drunk, huh? Who knew the Drunken Fist was so deadly...

Ah, no. There is a Bog Lurker FOE in EO 3 that boosts defense and life drains though. What I'm talking about is a browser-based RPG called 'Adventure Quest'. For the most part, it doesn't have too challenging enemies. The Dracolich Boss with Auto-Life is actually from here, though it's not really too threatening beyond its fear breath attack.

The most dangerous ones are the stun-lockers are the DoTers. In AQ, DoTs stack and the damage worsens from repeated inflictions. One really nasty case of the latter is Krenos the Prime Energy Dragon. His normal attack has a high chance of bleeding and he has an energy breath attack that deals massive damage and seals spell-casting for a few turns. He also has a chance to paralyze each turn by paying some of his MP bar.

They did design one boss that you can't brute-force though. The Wind Essence Dragon. It caps out the maximum damage you can deal it with a single hit to a low 200 dmg (it has like 13,000 HP). This means you need multi-hit attacks for full efficiency. It also has a passive effect where it steals 50% of all HP/MP you heal. And finally, it activates a buff called 'Wind Wall' at <50% which drastically increases its evasion so long as it has MP to maintain its upkeep. Its attacks itself aren't very damaging but it's meant to be a battle of attrition that's mostly stacked against you.

Also, before they were nerfed, the Syphon Vamps had a nasty bite attack that not only healed them a lot of HP but also level-drained you (along with some skill points from your highest stat). If you can't kill them in one-shot, you were kind of screwed since all your future attempts would gradually have decreased chance as your stats are drained and theirs increased.

Etrian Odyssey 5 is actually coming later this year... in Japan. :(
They have some cool-looking classes that I'm wondering what their skills are.
That series sure has vicious bosses and normal encounters. I remember getting swarmed by wolf FOEs and Ant FOEs. The latter is worse since helpers will infinitely spawn and slowly approach you and eventually join the battle unless you end the encounter quickly. Right now, I'm playing EO 3 and am in the third stratum which is a lava area. There are some rooms with Mother Dragon FOEs. Fortunately, they're not aggressive so they're ignore me unless I enter a random encounter in the same room as them. In which case, they'll start moving towards me; all of them. So yeah, don't fight even one Mother Dragon unless you can kill one before the others reach you.

You can also find Dragon Eggs in random encounter. They hatch into Dragon Pups in a random amount of turns. You can kill the Eggs for risk-free EXP, albeit less than a Dragon Pup. The Dragon Pups will cry for help and spawn a Mother Dragon if you take too long to kill them though.

Iwaopeln was one of the notable bosses in the first game that really stuck out to me. Though that was only because it would resurrect indefinitely so long as there was a FOE still alive on its floor.

A few weeks back, I fought Nebiros the Necromancer in Devil Survivor 2. It's a turn-based game where you move your units around on the field like Fire Emblem. Anyway, what was really cool about him in the second encounter is that he will switch bodies if you kill him, replacing the leader of an enemy unit. Combined with his Necromancy skill which revives all defeated units on his turn, makes him very hard to put down for good.

I found those to be interesting experiences but I suppose they don't really apply here since there's no FOE system or a tile-based war system.

Well, there was the Catfish boss Narmer who is the first boss in EO3. It has an attack where it dives underwater and then several puddles appear and you must guess correctly which one he's in (by attacking them). If you don't find him fast enough, he'll jump out and unleash a powerful Earthquake attack.