Fire Emblem X: Radiant Dawn.
I was in one of the later maps where the enemy commander was a female Pegasus Knight (maybe Seraph Kn). Now as you may or may not know, those units have pretty decent movement plus they fly so they go pretty much everywhere. However, the commander didn't move. Yay.
I clear the map and begin to surround the enemy commander with units. The plan is to let a weaker unit of mine get the kill. Good plan. I hit the turn end button and the enemy commander not only suddenly moves but also rushes a unit that can't defend itself and kills it.
*twitch* Okay, so the enemy commander suddenly moves on the next turn. I reload my battle save and move my units a little differently to protect that weaker unit so the enemy commander can't get to it. Good plan. I hit the turn end button and the enemy commander moves to a different that was low on health and kills it.
*twitch* Okay... so maybe I should move that unit out of her range and still protect the other one. I reload my battle save to do just that. I hit the turn end button. She attacks someone else, gets a critical hit (IIRC), and kills that unit too! Pretty much everything else on the map could kill her but this boss knew exactly which units to hit to kill and was stupidly lucky. Points for a good AI.
*twitch* It's go time now though. I reload my battle save yet again and send my commander to fight her. Critical Hit. Instant KO. It felt good to see her go down. I even got a 5 point level up off of her.