Just want to document this for myself for future reference (and for anyone else who might find an idea or two useful).
Only did the first quest so far; will update when I get to the later ones.
1. Gorr: Isophyne; probably restarted like 10 times until I did two things: (1) realize I didn't have my 5/5 MD mastery loadout active and (2) learn his SP2 dex. After that, I was able to get him down pretty easy.
2. Dazzler: Knull. Just used SP1s and only baited SP1s.
3. Lady

20 minute fight with FAM. Just used SP3s. Dash back when the power drain timer is about to expire.
4. Enchantress: Onslaught; easy fight because you benefit from her power gain and can special her constantly.
5. Thor Rags: Chee. Just bait specials constantly. Don't risk being aggressive.
6. Arcade: Vox. Easy fight because of undermine, poison immunity, and huge degen damage.
7. Leader: Attuma. My first time using Attuma; it went fine. Just stay close, bait only SP1s.
8. Bullseye: Sinister.
9. Torch: Spam. Need to read the nodes here to avoid dying, but otherwise easy if you're patient.
10. Serpent: Easy with R1 Spiral. Had never used her before, but I think she's great. That's the one benefit of this quest, made me realize that I need to rank up Spiral and put her in my BG deck instead of Isophyne.
11. Dust: Easy with R1 Ironheart. Another champ I had never used before, but really like now. Not sure I need or can afford to rank up another tech champ, but she's (or it's?) definitely a great attacker.
12. DThanos: Jean. Would've been a long and annoying fight without her.