Hey Dungeon Masters, curious how to run an encounter on this map?

The colossus is a fixed structure within the molten lava, notice the large chains. This stationary beast must be destroyed. The large eye is the weakness but it is protected both by some kind of magic field and minions running around the terrain trying to stop your players. There are 3 runic circles that must be broken for your players to remove the force field and destroy the eye.