Alright, so here's the deal that I'm needing help with. I have 6 lvl 14's that are storming a dungeon as a Session Zero one shot. This session will set up some of the overarching stuff going on for the main campaign's plot. Here's the setup, and my question:

Setup - A breach is being opened to the Abyss so that Baphomet and his forces have a stable portal to the Material Plane. It's being opened by a ritual being done by cultists. My adventurers are apart of the gathering forces sent to stop them. There will be two caverns, and then the ritual area. They're being assisted by 3 Demigods and a handful of mercenaries. Here's how I'm looking at it going down:

Encounter 1 - Scouts and shock troops

Encounter 2 - Shock troops and some heavies

Encounter 3 - Heavies with an additional wave happening that'll include Baphomet himself and 2 Goristros

My biggest issue is that I'm trying to come up with what type of shock troops and such to use. I can find plenty of info on the Blood War and such, but less so on what all are used as the grunts of the Abyssal army. What're their compositions like? Or do I just look through all the folios and find some cool demons to throw in? This is my first time DM'ing, and this session will set up the campaign. They're going into the session with the idea that these are pregen characters that'll possibly (most likely) die. What'll happen is that their forces just aren't enough to take everything on, they're going to get overwhelmed, and in a desperate bid trap Baphomet in a spatial gem while cutting off the ritual so the portal destabilizes. They're okay with the premise (its giving them a chance to try out a few things before settling on their own characters). I'm just lost in how to set up these battles.