If I look at the animations, I can find a couple ones possibly used for attacks. I remember seeing a "fall," "throw," and a couple other things. (I saw those with the ORIGINAL HL model, maybe the Hl:R one doesn't have these.) I can come up with gimmicks for those.

The throw attack fires a singular red ball that travels quickly and deals quite a bit of damage, but has a very small hit radius.

Fall happens when Nihilanth takes a massive amount of damage in a short time. He falls to the ground, and creates a large shockwave that disrupts people in a large radius around him.

Also, his green orb could teleport you as if you used the Displacer Cannon secondary attack. Maybe it'll also spawn a bunch of aliens around you

Displacer has glitches on some maps. For instance, you get spawned inside the skybox sometimes, etc.

Copy that on the harder Xenians, though. I might be able to replace some code to get rid of the errors on grunt spawn too.

I made a PAC that replicates what the Harder Nihilanth could end up being like (if all goes well) It's where I got the idea for the Throw attack from