Though this may be a simple sidescrolling shooter, it's one of the best games on the site. The combat works exactly how you would expect it to and the animations are some of the best out there. I'd recommend playing through it to win once, and then go back to get all the death animations once you've unlocked unlimited health/ammo.
For an added challenge, try getting a fly from the first level into the boss room
For an added challenge, try getting a fly from the first level into the boss room