Algodoo

Yes, I know the map is a bit bland but I just needed a proof-of-concept.

The boss here has 10,000 HP compared to the marbles' 100 each. The marbles can only do melee damage, influenced by a base damage stat and the marble's speed relative to the boss (I tried adding ranged attacks with a cannon but I could not get it to work with the shield.) The boss won't be able to do melee damage but will have a variety of attacks to choose from. Unlike a lot of bosses whose attacks are scripted based on the boss' health, this boss attacks dynamically based on an anger system, so the more damage the boss takes, the angrier it gets and the more likely it is to attack (though some attacks are more common than others.) Plus, its attacks get stronger the less health the boss has (e.g. shields have more durability.) I did add green and red circles in the middle, where marbles can heal and increase their base damage respectively, though.

Side note: For some reason the first time the boss uses the Poison attack, at least in my runs, one of the puddles just refuses to spawn, and this does not happen with all the other instances. So it's not completely bug-free.

Again, I'm not sure if this idea has been implemented already. If it has, I certainly have not seen it yet.

As always credit is appreciated.

UPDATE: fixed a bug relating to poison puddle spawning