My way worked much better. It was quite easy once i figured out the different waves of enemies.
So immediately I ordered my men to close off the passage on your right. If you just press LB + X one time, then it'll only send 3 men. So press that combination again to get 6 men over there, it'll go faster. Then just stay where you are and start up that chant to help them out. Finish it.
Now go up to the relic and fight off the first wave of enemies. The first wave is easiest because there are no enemies with shields. Keep in mind that there is a weapon rack of axes right near you, so watch for your weapon to break, even legendary weapons do break.
After the first wave there will be a little break in between, so get 6 of your guys started on the right boulder WITHOUT chanting. Wait until you get that prompt to push RB, which will turn beowulf to look out into the sea to see the next wave of enemies. After you gain control, start the chant and finish it. Then be prepared to fight right afterwards. You should of killed the majority of that wave of enemies with that boulder, but there will be about 6-7 that you and your thanes will have to fight off.
The next wave will include the 2 trolls. Again there will be a little break in between the waves of enemies. Command 6 of your men to start working on the left boulder WITHOUT chanting. Now this one you have to time just right. The RB command will flash, press it, look out into the sea. Then right when the trolls rise up from the water and start walking on the beach begin the chant. As the boulder falls you can turn around and see if it kills both trolls. It should hit them before they get to the entrance. If not, start sooner, or later. Bam, 2 trolls dead.
Then immediately after the trolls get smashed there will be one more wave of enemies. Call your men up to help you by the relic. Some will come some wont, dont worry about it. Use your heroic boost to help defeat the last wave of enemies.