Hint:

In the second episode where you have to defeat the catupult you can just choose to move and go to the other side of the map. There is a pionty rock near the ledge where the catupult is get on the rock and jump toward the wall if you do this right you can walk up the wall when you get close you can jump onto the ledge. You'll be safe from enemies they can't blow up the ledge. And if you shoot the ammo boxes next to the catupult and it'll do some heavy damage.