How do you beat Super Boss fights with no losses?

[Mystic Phoenix]

Another tactic which worked for me on Impossible Warrior at Level 30 (could have worked some levels before) was to bring Dryads and Royal Thorns (along with Paladins, Red and Green Dragons) to the spider fight.

With Time Back you have 3 huge stacks of spawned thorns, the first wave of spiders are nor problem to kill, so you can have 6 stacks of thorns very soon. With some thorns attacking the spider, additional spiders are generated not too often, and in case there are you might have no problems to kill them (attack with thorns first, so you take no retaliation) and so you can spawn even more thorns.
My troops didn't cause any damage to the Giant spider itself (red dragons used their breath ability from time to time), just leaned back and watched the show.