Successfully killed both KOAL and Red Dragon repeatedly with non-thrasher groups. Thrashers are def not needed for either boss, although they obviously do make it easier.

@ Crimson, difficulty of selecting and not being able to hold target is probably the toughest thing about the dragon. Rather than frantically trying to target him, grouping and going bananas with AOE seems the best strategy at the moment, along the lines of what Zheotare said above.