Balancing fight

Hi there.
I was making a secondary boss battle for my game, but i just realised that i can't know when the player will fight him, so i was wondering if there's a way (or a plug-in) to make the enemy weaker or strongher, based on player's level.

You can try out Yanfly's Enemy Levels

That let's you set custom levels for enemies to start at based on various conditions.

You can also use states to crank up or down its base stats since you can give or take away hard numbers. Maybe put some of its skills behind "party level" conditions?

If it's an evented battle (not a random encounter), you could set up several possible troops in the database with different stats.
Then, in the event you could set a variable equal to the party's level and use Conditional Branch to make them fight the appropriate troop.