I've been using the strat from Munkki's run of leaving the two NPC's outside and syringe clipping through the door and discovered that if you leave the NPC's close enough to the outside of the door, when you teleport to them for the syringe animation it instantly takes you to the next level.

You just have to make sure that you take Pierce's flamethrower between your own syringe animation and the one on the NPC.

I haven't done much testing with it, but after accidentally getting it to happen I tried again and got it first try, so it doesn't seem too hard to do. If you don't get it right then you can just continue with using Pierce to clip through the door.

I don't know if anyone knew this already, but I never saw it mentioned anywhere. I happened to press a number key to test it as a hotkey for LiveSplit and it switched my equipped weapon.

The weapons are also locked to the same numbers regardless of what you have on you, so it's much easier and quicker to select specific weapons.