Jormac
The pirate captain will revive his meat shields if either of them are dead, so you need to co-ordinate your attacks.
I recommend stealing once with Shannon and then passing turns until both Shannon and Ethan have at least 2 SK points. Then use Bone Breaker to get rid of the meat shields and let Ethan triple attack Jormac. Heal and repeat as necessary.
If you are finding Jormac hard: he attacks when when hasn't revived his shields for 3 turns; you can have Ethan defend and give Shannon defend as well if needed.

Sea Monster
This boss has 3 forms. Both the second and final form come with minions who should be dispatched as quickly as you can.
The second form and the cube slime minions will need Ethan's Guard Breaker to efficiently dispatch.

Psychic Tree
Preparation is key here. This boss likes to waste away your stamina to prevent you from launching a serious assault on him.
Hidden in the forest is an accessory called Beserk Immunity. If Ethan equips it for 5 battles he learns a passive skill with the same effect. Then Shannon (for whom attacking a turn is most wasteful) can equip the accessory. The tree's DNA when obtained with tentacle will make Olga immune to beserk.
Next in the boss's arsenal is Command Breaker: it will remove your commands one by one each use. However, it will always leaves you with one command left. Attack if you have it equipped, and if it's not equipped, you skill command.
His final move, Scramble, deals Stamina damage. If you have any MDF mods it's a good idea to equip them, as this will reduce your Stamina loss. Also pay attention when selecting skills as scramble re-orders them.
I find boosting Ethan's defences and damages with Shannon's Super Drug to be very effective. Good luck!