Asgard Boss Fight: Hela
This fight against Hela, on the rainbow bridge Bifrost, is a little weird.
Hela's touch is death, even to 'immortal' characters like The Collector- so don't go for hand-to-hand. Instead try to maintain distance while pelting her with projectile attacks (beams don't seem to work as well). But don't get too distant or she'll shield up. The ideal distance is just as she starts to glide toward you, death-hand outstretched.
Do enough damage as she'll send waves of clones to glide after you. Beams do work well against these impostors.
Keep alternating between peashooter attacks on Hela and mega-damage on her clones to eventually wear her down to zero.
This ends the fight, and makes Hela available to purchase- probably the most useful of the boss characters, and one of the most all-around useful characters in the game. You also get a point toward the Who's The Boss challenge