The Imperial Guard. Honestly, they're surrounded by genetically modified supermen, the same gene-spliced knuckleheads infused with evil warp-energy, planet-eating dinosaur insects, lethal space elves, killer robots and a load of other things that can kill them by just thinking about them, pretty much.
And they're just your everyday men and women, given overclocked laserpointers (or regular ol' guns if they're out of luck) and dressed up in what amounts to a cardboard vest, if their commanders care enough. About their only advantage is that they're alot of them.
And they hold the line, and they die standing. Sometimes, they win. A lowly regular human prodding you, the powerful mummy robolord, with a bayonette might be a bit pathetic. It's less fun for you when his eight hundred thousand comrades join in.
So, yeah. The Imperial Guard. The Sphess Mehreens win the battles (and get the pictures), but the Imperial Guard win the war. Or put it this way; the sphess mehreens is the Emperor's scissors. The Imperial Guard is the Emprah's lawnmower. And it's one big flippin garden to cleanse.