Mirrorback you just need to focus on DPSing a lot of hp before any spiders spawn on phase 2. After that, switch to a weaker weapon, or use a weak bleed to get raxx below 1500 life.
7:^]
Partly because RNGsus literally held my hand the entire time getting him, got araxxor down to 900 hp before the mirrorback spawned on phase 3 the very first time I tried, and yeah!