Dont press F1!
Why?
You can be invinsible with F3!
Yeah, that's correct, the mega unit option sends the health through the roof, you literally will see the bar fill up across the screen LOL
But they're not invincible. They can get killed easily when frozen, unless you keep pressing F3 while defending that unit.
LOL still, I've ran Tanya into a base with F3 activated and she never dies once after that, that's with the whole base trying to gun her down and me running like I stole something.