A fantastic animation and a brilliant mashup by a dope animator - Randall Glass.
I animated (Maya) all the characters after the :51 mark (after the flaming barrel). Stuff I did not animate: the huge hordes where you see tons of zombies together on-screen at once (that was just L4D NPCs), nor did I animate the legs of the Headcrab zombie, or the crawling torso. Those were game animations. I animated the Hunter, but based his crawl on Valve's animations (I wasn't able to import those movements so I had to recreate them myself).
I created my own blendshapes/morphs for the Witch to make her talk and emote. The L4D version just has her do a couple things (an expression or two), but I wasn't able to use any of that. I also created my own rigs for controlling the zombies/infected in Maya.