I think the best idea would be to make an enemies group, and have megaman be in a players group. Then, make it so that the bullet
When this collides with something
If group for this actor's colliding shape=Enemies
Kill last collided actor
(I think that should work, but I'm not too good with the actor's colliding shape part-But I used that for spikes and it worked, so it'll probably work here too).
And by the way, but the enemy sprites in the Enemy group, and the bullet in a new one called bullet-Have the bullet group only collide with enemies and tiles.
And by the way, if you want to have a bullet take out, let's say, half of the life of the enemy, you can simply add the health behavior from the forge to the enemies, make them have, let's say, 30 life, and NOT invincible, and have another one in the bullet, Inflicts Damage, and have it take out 15 or whatever you want. Make sure the bullet and the enemies collide, though.