This is a elongated solution that spells things out a bit more, but here's the Cliff Notes version.
-- The Raven's Spire fight with Loki will not unlock the achievement.
-- Going through the Portal back to the Warrior's Hall will allow you to collect four God-Swords to free the Destroyer Armor from its icy prison.
-- Loki will reveal that this was a trap so that HE could use the Destroyer Armor all along and will disappear into one of four portal rooms
-- Guess the correct room that he's in, draw the Destroyer Armor towards you--he'll follow you in, and then deal damage to the Destroyer Armor so that it self-destructs. This will defeat Loki, achievement unlocked.
You will have two fights with Loki:
The first will be in Raven's Spire, which shouldn't be too difficult if you use ranged attacks and kill the enemies that he keeps spawning to regen your energy. I had Deadpool beefed out at this point and he was absolutely destroying him no problem.
The second battle is a bit more tricky, you'll teleport to the Warrior's Hall where Nick Fury will task you with finding the four God-Swords necessary to free the Destroyer Armor. Behind your characters will be four portal rooms, where you can enter and go through various puzzles to get each sword. Most of them are straightforward, and the only one that isn't is the one with the dancing sword, and I'm not sure if I can explain how to beat that one, I think just going into portals where the sword isn't going is a good strategy.
Once the four God-Swords are collected, Nick Fury will reveal himself to be Loki, glad that you did his dirty work for him. THIS is the battle that will unlock the achievement. Waste no time with the enemies in the main hall and immediately try to track Loki down as he disappears into the portal rooms. These rooms are not the same as the God-Sword rooms, they are simply a one room affair. If you see a yellow blob inside of a crystal in one of the portal rooms, you've found Loki, if not, you'll need to defeat all Loki Trolls before opening a portal out of the wrong room. Once you've found Loki's hiding place, draw the Destroyer Armor close to the crystal, baiting it along and then blasting it with everything you've got. It will self-destruct and massacre Loki inside of his crystal. Careful though, the game glitched on me when the Destroyer Armor picked up a barrel to throw at my team. We overloaded him, but he never did the self-destruct move because he was too busy trying to unload a barrel, and I had to reload a saved game.