Oh! My most favorite topic! Let me share a few fun ideas for bosses
Idea number one: Countdown to total destruction
One of the basics. Used few times in Final Fantasy, mainly when boss was Bahamut. Each turn you showing message (or animationless skill with specific number for name) on how much turns left. You can make it by events pretty easy. After countdown reaches 0, force boss to use some uber skill and BAM! Battle is much more intense!
Idea number two: Flip-a-coin
You can make boss a skill which effect is random. This can be made by easy work with variables... Or without any! I'll say how without any: make boss few skills with same name, but with slightly different animations and effects. Example i gonna made in my game:
Charon has a nick for coins (sorta like Two-face from batman). One of his skills involves throwing up a coin. I made two different skills with same priority and same names and same cast animation. But when one skill deal heavy damage to party, other damaginfg Charon himself!
When technically it's 2 different skills, it's crating illusion of one skill to a player and adding tention to the battle
Idea number three: Copycatte
This shall require some work with events. Make boss to get all stat changes player shall cast on his party!
Make a boolean with condition "if actor have cond", make action "add cond" with that condition player have. And BAM! Player who always depended on buffs is screwed