I've been trying around, finding some glitches in the game. The probably best one for speedrunning is what I call "roof skip" Video: .

Basically you can jump ontop of a vending machine and get out of bounds and skip most of the roof, including the sniper section. In the follow up the game is in a glitched state and recognizes most triggers as activated, including the boss fight (which is already done, once you get there). However, since Zombeer is build very linear, it doesn't register you progress, is not setting save-points and activating scrips aside from cutscenes. So it won't load the end credits, making it impossible to finish the game.

Aside from being an interesting glitch, I'm wondering if anyone can make it actually work. So far I had no luck, but maybe someone else has an idea.