Actually, if you're fighting an irregular war you need, at least, to take out ten of their numbers for every 1 of yours who dies, to have a decent chance of winning the war. Although, if the irregulars have the complete support of the general population, you might as well call it quits and throw in the towel right at the start. Whether they are well armed or not doesn't really matter. They can do all the damage they need with home made bombs, AK47s and RPGs.
20th century irregular warfare is hell for conventional armies.