HOW CAN I MAKE A COMMON EVENT TRIGGER FOR ONLY THE USER OF A SKILL

This is worded quite vaguely, but I'll try my best. I guess what you want to do is have a character cast the spell, then have it trigger a Common Event that changes that character's equipment and a variable depending on who it is, right?

You can use the following trick to achieve it: Set the skill's target to "The User", then have it inflict a state that has no effects at all (I'll call it the "No Effect State"). This state must be set so that its infliction never fails and it is invisible to the player (priority = 0). The point behind this state is that it "marks" the character who used the skill.
Then, set up the Common Event that is triggered by the skill. Put Conditional Branches in the Common Event that check if a character has the "No Effect State". You have to make one Conditional Branch for each character that can be in the party. Inside those Conditional Branches, set the commands for whatever you want to change about the respective character (in this case, equipment and an individual variable). Don't forget to remove the "No Effect State" from all characters at the end of the Common Event. Then it should work fine.

I hope this helps.

You're welcome. I'm glad it worked well.