I finally managed to do this part on Legend/carnal after much frustration, using the methods suggested here...defo hardest section of the game.
For me the deciding factor here seemed to be killing both trolls with one boulder. I stood on the platform where the relic is and ordered my men to shift the boulder, timing it so that both should get hit. Even if one survives, its a pain in the ass killing it quickly enough to get ready for the final wave.
Other than that I kept on the walkway to the platform and just freakin mauled on anything that came by with combos; if anything got by, I turned off carnal and just tried to take their attention off the relic.Using carnel on that little platform unless you're real careful is asking for trouble.
Retail Games with 100% completion : 50
Arcade Games with 100% completion: 7
Currently looking to boost; Condemned 2, Crackdown co-op
Info for potential multiplayer pals; I am a UK brah so go by 'GMT'