The Sentinel is impossible as Rath?

I played Battleborn yesterday as everyone else did and decided to play public campaign and I am incredibly lucky that the two people I paired with were both Marquis. Is there some trick to damaging the guardians and bosses in this level. I could sometimes damage them by aiming at their chests which I think were their critical spots. I felt practically useless except for the occasional thralls. Am I just crazy, a trick I missed, something? Anyone have any thoughts?

The guardians have armors that protect their chests. You have to shatter the armor first, then you can damage them.

He means for the final sentinel. The final sentinel is so tall that it is very hard for Rath to melee his chest to do damage. He can use his one ability to do some damage to it but that is it. Pretty much ends up just killing the small guardians and having to let the rest of the group damage the main boss.