Oh my GOD this level. Zombies are the absolute worst enemy for speedrunning for pretty obvious reasons, luckily there's one small exploit with them where you can leave rooms before a corpse reanimates if you're quick enough, and with very rare rng of a long wave delay, allows skipping an entire wave on the 2nd stairs. Even with that sub 2 was ridiculously hard, I'd recommend giving it a try if you're looking for a serious challenge in this game.