Doing Zombie Chopper right now. I'll tell you a little trick that I found out: bringing Dog's "ball"/disarmed Rollermine from the previous chapter helps a bunch, as it is a persistent (non-exploding) projectile that will seek out and attack zombies and headcrabs. It can't do anything but distract zombies (a useful thing in and of itself), but it will kill headcrabs if it runs over them. Kind of tricky to bring it all the way through the level, but it is well worth it. Just don't let anything explode near it or bring it underwater, as those are the only things that can destroy it (i.e. it's safe to let it keep the zombies busy). Also, it follows you if you aren't holding it, which lets you hold another item and instantly have it in reserve.
EDIT: Oh, wait. This thread's been dead for a year, not a month. Misread the year. Sorry.