In all the talk of graphical downgrades no one seems much preoccupied with 'why?'. Why build something and then proceed to tear it down, piece by piece, in the hope that ever more diminished expectations about the final product won't be severe enough to...
Don't stay in one place for too long if there are snipers about.
You can jump back towards walls you've already jumped off to avoid obstacles.
In the end-of-level scraps, take out anything with a gun before you deal with the others.