Planetar

8) When asking the planetar for help, have him grill them about their past deeds and/or hypothetical situations, to see if they’re worthy. He can tell if they try to lie. (Winning is spinning the answer to their advantage without outright lying.)
9) When hostile, have the planetar use Blade Barrier to isolate one PC, so it can duel and kill him. In mountains, Blade Barrier is especially nasty, as it will sever ropes and impose disadvantage on climb skill checks.
10) Control Weather is also a powerful spell to use in battle. You just need to set it up in advance. Bring down strong winds when climbing mountains, or a flash flood in a canyon. In addition, the planetar can send Flame Strikes against them as they try to reach safety.

Solar

11) For a plot complication, have the solar (or any other angel) insist on taking out a certain, less important villain. The players know that doing so would be a mistake (morally or strategically), but the angel won’t listen. If the PCs won’t do it, he’ll send someone else. What do they do?
12) In combat, the solar will use Control weather to fill the sky with scattered clouds. (With 25 INT, it’s not caught by surprise). It its turn, it will shoot, make a flying sword attack, and go back up. In between turns, it will teleport around, so they won’t know where to send area spells.
13) For an absolutely deadly battle, have the players be trying to beat the clock in a trap dungeon while fighting a solar. The solar can teleport to always be in the way, it can avoid many trap effect by flying, and it can blind them. Use for really powerful players only.