Well good game.

Bring on some little or stylish explosions when ennemies are hit ! Why not exploding in cubes ?

For collisions use simple oriented cube collisions, or why not
sphere collisions it should work very well also.

I think ennemies should ignore other ennemies ,no collisions
and the yshould also ignore other ennemies fire caus it's not
an FPS game with AI !
For the bomb of player , only ennemies should feel it !

I suggest you also to make a shader based or brighter laser effect when you shoot, caus we don't see it a lot.
Make it more big, caus it seems to be too thin , and it's hard to have to target very precisely each ennemy and the games goes fast.

Well juts my opinion, keep it up !