Firstly, to get to Evelyn you go through the Robots cave and out the other side.
I assume you mean the room directly before Evelyn with lots of spaced out casters and such, I'd recommend you hightail your party into the room and then go around the corner giving some line of sight defense and such - the cultists are not especially tough depending on what level you fight there, as level 5 you could probably be level 6 before that if you completed enough side quests of appropriate level, such as Black Cove or Pet Pal activated ones.
Control is the key here - Jahan has freezing spells (if it's raining, his 3 point chill will freeze them), your fire mage can hopefully teleport a mob where needed if they get too close/teleport into madora's swing. Your rogue should pull out a bow and focus on sniping things so you're not so exposed to everything in the room - also feel free to use the level 4 charm to help turn the tide.
It's quite a tough fight if you do it in the open, but if you focus on controlling the board it's much easier.
