During the fight with red Skull you will control two characters: Human Torch and Captain America. At the beginning of the duel, cover yourself with Captain's shield and approach to the enemy, so you can punch him for the first time. Then return to the central part of the arena and fight with some minions.
When the stage is partially clear (notice that enemies will appear endlessly), turn to the left and search for some bricks scattered under the cannon. Build a shield switch from them, then toss a shield at it. Because a golden wall is uncovered now, you can melt it with a fire beam and destroy a gas cylinder hidden inside. In this way you will damage the first gun.
Go to the wall at the opposite side of the arena and look for a lever. Use it, then melt another golden wall, blowing up another gas cylinder. When the second cannon is broken, you can switch into Captain America and hit Red Skull in the face (remember to take cover behind your shield!).
Third stage of the fight looks very similar - the only difference is a reversal of the sides of the arena. That means you should destroy two new cannons, acting in the same way as it was described above, but at the opposite sides of the room.
Finally, you can strike the final blow on the enemy.