Fast speed, high all around attributes, low status resistances, as standard for a cavalry unit. Angel Knights pack a powerful punch, and move fast enough that they have a good chance of delivering it, so long as they aren't up against Freeze or Shock units.
Good Against:
Aside from earth units, Angel Knights' fast attacks with solid AOEs make them good for handling small units like goblins.
Bad Against:
While Angel Knights take 20% damage from Shock causing units, making it unlikely to kill them, they have little actual resistance to the status effect itself, making Freeze or Shock ranged units in general extremely effective against them.