You suffer the same problem I used to have: You stay in the middle platform. It is suicide to be in the middle because as you said, Rendain will knock you off. However, if you go to the right platform (The one where the Conservator normally spawns. The icy platform) you will be pretty good. Yeah, the thralls kinda suck, BUT THAT IS THE POINT! If you can draw Rendain away from his million thralls, or kill the thralls while also distracting Rendain, it is easy. The Heliophage is a pretty easy map to complete, and I have done it many, many times. You just need to stay off the middle platform and kill the Thralls. If a teammate dies to the Thralls, don’t help them until the Thralls die. You will die yourself. Leave them be and let them use a life (which at this point, you should have 10-13).
Let me say it again: STAY OFF THE MIDDLE PLATFORM!

(Though this can be ignored if you just want to get the mission done)