To be honest, we have no idea, nor does anybody else because this will be the first of the updated Challenge Events where the mobs, quantity of stages, and end bosses and all stats are unknown.
That said, my personal prior experience is a handful of fast snipers is more than adequate to get the job done, three in each color. If you want to be a little more versatile for future events, have maybe one or two Hawkmoon and/or Belith just to cover all bases. There are specialist heroes which you should keep. They’re not needed for challenge events, but they might be useful to keep around if you’re interested in taking part in 3* tournaments at all, but that encompasses a much larger range of heroes that you may or may not have the roster space for. Generally any spirit linker should keep one set of as those are rare in general and can be quite useful for helping team survivability.
So starting in elemental ordering (purple-yellow-blue-green-red):
- Yellow: Kailani (1), Melia (1). Save any future Banes but 2 Banes + 1 Gan Ju should be a more than adequate 3* yellow team.
- Purple: Cochin (1), Gill-Ra (1). Save any future Balthazar pulls until you have 3. If you get a Tyrum save one as he’s one of the few non-green dispellers which can be useful.
- Blue: Valen (3), Gunnar (1), Gato (1-3). If you get an Ulmer, keep just one.
- Green: Berden (1), Belith (2, save both), Mnessus (1-3), Brienne (1). If you want to be somewhat more versatile in 3* tourneys, Carver and Tuck can be supremely useful but aren’t that useful in Challenge Events in general.
- Red: thin pickings here, but I’d focus on your Nashgar (keep both), Hawkmoon, and Azar.
If they’re not listed, they can be fed.
For Knights of Avalon, I’m not sure what you can manage as you have a little over a week at this point, but assuming you won’t have any trouble leveling up multiple greens, Mnessus and Belith or Brienne should be the priorities, depending on if you’re a more aggressive or defensive player (Belith better as a defensive player, Brienne for more aggressive players) as you may or may not be able to swing it with healing battle items without a healer.
Blue is not the priority due to Avalon being a blue reflect, so Gill-Ra can be quite useful as the only 3* cleanser at this time and the defense down can be pretty handy. Kailani in yellow because the spirit link could be critical for later stages if you need your team to survive, and red will probably be weak in Avalon due to the abundance of blue mobs, but either Nashgar or Hawkmoon depending once again on your playstyle.
So what I’d recommend for immediate leveling priority:
- Belith or Brienne
- Nashgar or Hawkmoon
Assuming on-color feeding with a mixture of TC1/4/19 and TC2 depending on resources. Should be able to get those all up by the time Avalon hits, depending on resources.