I can't work out at what point you take damage from the enemy, is it when they reach the centre of the screen?
Look quick, there will be a flash from their guns, indicating they are firing at and damaging you (note: that's in the second to the last video, at least; I haven't seen the most current one, so maybe it's not as obvious?).
Thanks cNp. VectorX is right, there is a flash when they shoot and when you get hit, the everything flashs. It's like in modern shooters, there's no "point" or something that shows the Bullet. Every Enemy got a hit rate that increases after every time you destroys a tank. The only way (besides luck with the hit rate) is to duck.
The tank shoots rocket that can't be destroyed. You need to duck before they scale to the biggest sizes. If you don't duck, you lost a hitpoint everytime.