Weeeeeel, luckely I've been a bit more fortunate in my quest for a lvl 4 Boss guide.

Here's a rap up:
First off, the boss will not fire at you as long as you are standing on a moving platform on the 3rd alltitude. To kill the boss you will, however, get him to fire at you. When the boss fires you must try to jump around and guide the missle back at the tank. That will kill him.
The strategy is pretty much the same as the one you had them hit the force fields. Remember to keep on moving about.

Good luck!