So I've started to run Zeno Clash a while back. Despite running on source, it's very different from hl2 as it turns out. For one, this is not even a game where you can jump so there's a lot of movement possibilities lost right there. Good news is, movement is not even that important anyways; a lot of levels end when a certain amount of enemies are killed, so it's not like you're racing to hit a loading zone. As far as I can tell, speedrunning this is all about optimizing fights, optimizing scripted events (can't tap npcs on the head with props but you get to influence their movement at times). There's also a couple skips that's I've found along the way.
I've been working the game down and finally got a sub 2h run, it's not perfect by any means but I think it's a good starting point. you can watch it here https://www.youtube.com/watch?v=hZ5A_iyBrDQ
it's honestly not a great speedrunning game, you can't skip cutscenes, 10min autoscroller right in the middle but it has a couple interesting moments.
But yeah, if anyone wants to tag along in helping me breaking down this game that would be great. But remember, there's no jumping and no picking up items so we have to get creative here.