The lines when you set up an AT gun is the firing arc. If you are outside of that it can't shoot you (this rule doesn't apply to machine guns for some reason) after you take the first hit, move OUT of the arc instead of remaining in it, your units will survive much longer. Exit arc stage left, not backwards. You can thank me later for revolutionizing you game experience.
Unless the halftrack got some kind of speed buff added to the flame upgrade that I'm not aware of, it isn't capable of moving out of an AT gun's arc of fire in time to avoid the second hit even if it starts moving within half a second after being hit due to its horrific acceleration, nightmarish turn speed (which comes into play unless you miraculously give a move order directly in front or behind, which barely ever happens) and low movement speed overall.