Solo it, or duo it with a good Eir. Hellios always comes from the right side after the second thing breaks, so typically I just do this:
1. Break left, ignore kanhel, kill dragons in middle.
2. Break right, obliterate Hellios in 2 seconds thanks to lolsolohp, kill dragons in middle.
3. Break down, kill dragons once again.
4. Break up, lure boss to a side, die on purpose to obliterate it with a 501+ str aeh'd rb impartial.
All the while kanhel would be flying around being stupid.
In summary though, there are 2 reliable ways to beat that stage. One, having a full DPS party and just have every kill everything as soon as possible. Two, have an Eir/Krieg spam heals while the rest of the party go out and kill stuff.
...And don't do random parties. Much easier to solo than to do a random party imo `-`