by brushing against them

Always keep moving, never stand still. When you push forward, you must always be ready to shift into reverse at the same time.

try to destroy those large ships

Be brave when fighting the bosses! They release their sparks and waves in intervals (you can observe the timing). When they’re neither emitting sparks nor sending out waves, get really close and fire, fire, fire! When they’re hit (they flash white with each hit), the bosses don’t strike back immediately. You just have to watch out that they don’t suddenly leap toward you (but you can see that coming too: they briefly pull back first).
You can do it!