I had some trouble wrapping my head around Gambits the first time, and it seems my understanding hasn't been improved upon this re-release.
What's the best Gambit combo to have my controllable party leader focus on a target without going off to target other enemies randomly? I'm trying to set it up so that Vaan only attacks enemies that either aggro him or his allies, but this also results in him dancing around multiple targets per battle.
Put a "Foe: leader's target -> attack" above the specific one you wanted to ensure the leader will keep targeting the same enemy they targeted on their last "turn". Gambits are easy to figure out when you think of them as checks the character goes through from top to bottom every time they're taking an action, and if something's a match, they execute the action and start again from gambit #1. With the following gambits on your leader:
#1: Foe: leader's target -> attack
#2: Foe: nearby -> attack
(I don't remember the exact names, but you get what I'm saying)
On the first "turn", you (the leader character) haven't targeted anyone, so gambit #1 is skipped and your leader picks a target according to the second gambit - the nearest one to them, enemy A.
On the second turn and every turn after that, #1 ensures your leader won't switch targets, cause at the moment the second turn started, they were still targeting enemy A. Thus, gambit #1 checks out and they attack the same target again. And again and again and again, until the enemy dies, after which they again pick the nearest one according to #2.
Unless you want to get fancy with weaknesses and such, I just use the following
#1: Foe: leader's target -> attack
#2: Foe: least HP -> attack
as a baseline on every party member. Leader picks the target with the least current HP and starts attacking it and the rest of the party follow. Even if the leader isn't targeting anyone (i.e. #1 is skipped) everyone will pick the enemy with the least HP, so obviously everyone will attack the same one.
e: it's 2:30am so I hope this makes at least some sense lmao. Long story short, just put "Foe: leader's target -> attack" above the rest of the regular attack gambits on your leader and they won't jump from target to target all the time.