Completed another one today, Alien Shooter. Playing it reminded me of Smash TV on the sega that I used to play. That game and this just throw 100's of enemies at you as you hold your gun trigger down and dance around them trying not to get hit. I think it only took about an hour and 30 mins to finish it. Fun little distraction but once you get the final gun the last two levels are way too easy.
I didn't find the final boss easy at all....
Anyway, try Alien Shooter 2 next, a significantly better game IMO.
Edit: Just finished Spectromancer on hardest difficulty - damn that took a lot of retries.