Hi, were in this terrible pug but we made Flame Leviathen after 5 tries. After every try he didn't reset so when we zoned in for the fourth time, he still had 20% hp.
So how to do this is to pull Flame Leviathen back, behind his zone, so you will be fighting him where you killed all the pylons and shit. Guess you can do this with 25 man hardmode aswell.