Asisreo
Patron Badass
So my initial plan is to, quite simply, tough out any damage they deal and to attempt to hit them back kinda hard.The one in the OP with the green flying creatures.
Now, if I'm playing a Battlemaster, the very first thing I'd attempt is a Trip Attack to prone the target. Its flying so my hope is that I can get additional damage via fall damage. It still resists it, but its still a decent chunk for each creature.
Next, I would probably try to disarm them. I wouldn't have known this from the description, but they don't have proficiency in either Dex or Str and they don't have advantage since they aren't resisting magic.
Now, this could be unacceptable to some DM's, but by RAW, it is possible to pick up a monster's weapon. I might not since they're not likely to engage in melee without their weapons anyways and I'd have disadvantage due to its size, but its something I'll keep in mind especially since they are magical and bypass resistances.
But perishing that thought, my gameplan after that is to just take any more abuse they can muster and hope they can't debilitate me with Wisdom/Intelligence saves.
On the Planetar's side, I'd assume they'd try to close in for melee attacks since that's their strongest option unless there's a party in AoE distance. They can't disable the fighter easily, but they do still hit extremely hard when they get in range. They'd probably avoid being in the air after their turn, though, to avoid taking unnecessary fall damage.