I'm with you on GAS: VERY painful. Check out my post: Phase 4 Mods: it really IS all about RNG & the mods. https://forums.galaxy-of-heroes.starwars.ea.com/discussion/238221/phase-4-mods-it-really-is-all-about-rng-the-mods#latest
Check out the post as I now has GAS, thanks to the help of this forum, YouTubers - but mainly to my Guildmates - Game of Clones: First Legion. BIG lessons:
P4 is all about the opening salvo, just like the Chewie event. If they Daze, Stun, or go after B2 or B1 and Magna doesn't taunt or counter, RESTART: you'll never win since Magnaguards counter is critical. If they go after Magnaguard and he counters, then you have your shot. Otherwise, back out and start over.
Stunning Rex is a MUST; in my experience, you will know RNG is with you when you take down GAS w/out using Droidekas middle ability (Reconstructive Revolution), saving it to one-shot Fives. THIS is one most videos don't pass along but I found it critical to conquering Phase 4.
Good luck!