You mentioned the "the player should be able to avoid any damage as long as they keep moving and are paying attention" goal. In my gaming experience, the most effective attack from the enemies against me was like this. I was fully focused on the enemy on one side, preparing to fire from a distance first. At that moment, my vehicle was very close to the other side on the screen. Then, a small vehicle rushed out from the edge of the screen on the other side. Due to the close distance, it instantly approached and hit me.
I originally thought there was no need to mention this because according to what you said, "keep moving and are paying attention" could handle this situation. The reason I was hit was that I ignored the incoming warning from the other side and also because I tried to stay in place and fire. However, looking back, I was hit by this kind of attack too many times. Even from a statistical perspective, it was necessary for me to specifically mention this.
The other situation was the crazy attack of the enemies in the sixth level. A large group of enemies, including helicopters, forced me to retreat. Then, a small group of enemies surrounded me from behind.