Far better than I expected!
From your comments, I thought this was just going to be Space Invaders with different graphics. So the creature briefing raised an eyebrow and when the 2nd level started, it was obvious you'd spent a little time making this stand out a bit.
Unfortunately, I think a lot of people may just play the first level. It's a level which suggests the game will, in fact, just be space Invaders but with a slightly different movement pattern.
It's a game with a lot of thought obviously put into it.
It should be more obvious that we don't need to kill them before they reach the bottom of the screen. That messed me up a few times until I realised - when it's meant to be similar to Space Invaders, most folk will assume the same rules apply...
It was enjoyable for the most part - after playing through on Very Easy, I went back and did it on Very Hard.
The only bit I don't like is 'Mike' - there's a safe spot and it seems to be more a case of nipping out every so often when you think it's clear and getting a shot in, then going back to the safe bit than any real strategy.
A final boss should really have multiple attacks and multiple stages of destruction, but even disregarding that (let's say you couldn't do that for whatever reason) you should have had it easier to navigate in the middle, but with no safe spots.
It just seemed to be really repetitive that last guy, doing the one thing 8 times (or however many).
Anyway, got a large negative number on my first go, then 22905 after finishing on Very Hard.
Enjoyable game and I look forward to more from you.
Stay funky,
Bez