If this can help.
I had a lot of trouble killing the Chimaera, because I killed the troll first.
You should harm the troll so that the Chimaera lowers its shield down to heal it.
When it happens, use the ballistae to hurt the Chimaera.
Maybe rinse and repeat. I do not know how many waves you need to kill the Chimaera.
The troll itself is fairly easy to kill.
Make good use of the minions, and keep replenishing their lives by taking the new medals when they appear after kills.