During the fight with Loki you shouldn't bother yourself with frost giants - they will appear endlessly, so killing them will be a waste of time. What is much more important, you must break all objects in the area to gain bricks - they are necessary for building Captain America's panel in the centre of the room. Use it to reflect the fire beam from the Destroyer to Loki (see screen above).
When the God of Lies is burning, the Destroyer loses his invincibility powers, so you can easily hurt him. Use Thor to do that - only this hero has the proper strength.
Wait for an ice block to appear, then melt it with Human Torch and build another platform for Captain America from the debris. After that act in the same way as before - you must repeat all sequence for three times (in total).
In the last round, Loki will summon some doppelgangers - switch into Wolverine and sniff for the true enemy, then hit him. Now the fight is over.