Easiest solution, make the Imperial Guard Regiment(s) Elysian, Death Korps, Armageddon Steel, or Cadian Armored because all of those have high veteran units with elite gear. You can easily say in and out of lore the Death Korps wouldn't give a single #@!% about who they were facing because they would march into the Eye of Terror if ordered to and not think twice about it.

The Elysians have carapace armour and high damage, fast mobility weapons which is perfect for this game since they could receive the ability to basically aerially insert reinforcements on target because they're paratroopers basically.

Armageddon Steel Legion are basically the Death Korps but less creepy in some scenarios so you got another good option there.

Cadians are basically the poster boy/girls for the Imperial Guard and we all know they're easily one of the most loyal and fiercest regiments out there and have some of the most specialized regiments to date.

Now this isn't to say other regiments could cut the job, these are just the ones I can think of that would easily go toe to toe with any foe and have a decent chance of coming out on top....except for Chaos sorcery.....that's just OP and realistically, no one can fight Nurgles plague unless your name is Ursarker Creed or a Psyker.