I randomly found this other place you can boost besides the last level. Its on the 2nd level where you're at the very end of the mission after escaping from the area that's on fire and has a bajillion lickers coming at you. The lickers spawn infinitely (I think) but only like 2 to 4 at a time. But the lickers isn't how you can boost.
Once you get outside into the courtyard area where you fought spec ops earlier, there will be tons of zombies and they constantly keep spawning from the left and right of you. Once you kill them all they immediately respawn and crawl out from the left and right sides all over again and there is an infinite ammo box there so you can just blast away at them. The good thing about this is that they're the regular zombies. Not crimson heads like on the last level.