Ok here is the thing.
If you engage the boss on hardmode while having the whole raid standing outside the big doors you dont have to deal with the extra stuff he does inside the room.(Becouse you wont be there:P)
So:
1. have the raid stand outside the gates.
2. Someone engages the boss and also run out
3. Profit
Can bug sometimes tho=/
(Dont have any pics becouse i was only talking to my friends on vent while they were doing it)
GL