Oh wow. The last mission is dead simple with two time stoppers, two heavy cannons, and a damage buff. Slot the buff in the bottom right, time stoppers in the back and top, and cannons in the front and middle. Upgrade the middle cannon's damage, the frontmost cannon's health, and duration on the stoppers. Increase the damage buff as much as possible. After a few levels, you should be able to pretty much keep the enemies frozen in place the whole time.