I have an idea for one. This particular enemy can be any other type as well (usually combining with a type that makes them harder to kill) but uses one particular trick. It's attacks deal no damage up front and its immortal until it reaches 1hp. Instead, its attacks add a state that "stores" damage (and the player cannot see how much). It also tends to be a high agi enemy with more than one attack (or action) per turn.
When the enemy reaches 1 hp, it deals randomly between 50% and 200% of the damage it has stored in the player's states before it dies.