When I think about it, pretty much anyone from warhammer 40k would have no trouble with conquering hell. There are so many orks, hell would literally run out of space before all the orks got to fight. Same with the imperial gaurd (and COMMISSAR STEVE), and the tyranids. The necrons are close enough to being impossible to kill and there are enough of them that the same thing would apply. Besides their also overwhelming numbers, the tagline of the Tau is "yeah, we suck at close combat. Too bad you'll never get there". Even the eldar have a tried and true approach: guns that shoot shurikens and lightning.
The space marines wouldn't fill up hell, but think about it: The base space marine (ignoring specialist units, like terminators) stands 8 feet tall, can absorb blows that would reduce standard humans to bloody chunks, and weilds weapons that are generally mounted on vehicles, the least formidable of which is a grenade machine gun. And that's before he puts on his armor, which enhances his already formidable strength and makes him more or less pokey-thing proof, airtight, and fireproof (except for promethium, which te demons don't have). Then there's the grey knights. Plus, I've got this image in my mind of an imperator class titan accidentally squashing satan as it walks around the front line.
If I had to pick a non-40k army, I'd go with the Terrans from Starcraft/Starcraft 2. I'd fly in in a battlecruiser, vaporize satan with what is essentially a channeled nuclear blast, then watch all the demons turn on each other. That, or I'd just wage a conventional ground war.
Heck, the regular human armies of the modern world could do it; just go read Armageddon???
rigins