I have don't the colosseum challenge multiple times now and still have not got the triumph for rivensbane. I know we completed it because one member of our fireteam got the triumph while the rest of us didn't. We completed shuro chi multiple times after that to try and get the triumph completed and a couple of those we made it through without taking a hit or taking splash damage from her bolt. We even used the lament cheese and we still didn't get it. So I'm really confused what's going on but it's the last thing we need to get the rivensbane title.