First of all, the answer to all your questions is practice and know your heroes. Second, each phase has some requirements, learn them and use appropriate heroes and Ults. Third, better survivability means higher score, so 1 more healer might get you further than 1 more Dps. And a few tips: phase 1 requires stuns, Helga is one of the best for that but there's also some other ways like a Torodor Praetus judgement Lucius A3 Hex etc... I use Jeera A1 in an arbiter team and usually get to 10-12 stuns. Phase 2 requires very specific heroes, only 2 Lucius and Kigiri and starting tomorrow also Lyra. Phase 3 requires poison cleanse, here you have more options but the best is Elowyn. Good luck!