Io can stun Terminus by Luna through his siphon. Damage she deals from weapon is higher than from calamity blast and if she bought nimmble she is uncatchable without Terminus' cards on speed. Without his assistance you shouldn't be afraid of him.

Atlas is high damage tank very dangerous on solo fight. He can undo all dmg he has dealt for quite a while. He also can cancel all healing (self and from others) you've gained. To counter him you should always be with an ally to put pressure on him.

His insta heal is big, but only when ally is in his circle. Firstly you and "your allies" should pay attention to "his allies" and them occupy with something. Then when Grover is left alone it's your chance to challenge him on solo fight.

Androxus is great at short distance but he also can do much from long distance. The best way to counter him is bait him to use Reversal unnecessarily and force him to waste all Nether Steps and just stay grouped up.