See, that's the trick. People keep talking about entire parties of 20th level characters. This is something that should be seen maybe once every few centuries or so. Yes, if your campaign is really high level, then masses of 1st level warriors aren't going to be very effective. But, then again, if your campaign is really high level, then you don't get masses of 1st level warriors, you get masses of 5th level warriors. The whole balance shifts upwards if the campaign is set that way.
I wouldn't be so worried about the high level characters anyway. As I said, they should be extremely rare in practice. However, monstrous troops should be seen fairly often. It wouldn't take a tactical genious to realize the advantage of a giant over a catapult. Much higher rate of fire, far greater accuracy and much more mobile. Open up the treasury and hire half a dozen stone giants and you have a pretty effective artillery unit.
Or, within a very short time, a kingdom could build itself a fairly decent array of beasties to use. Remember, these don't take any technical knowhow to create, just some animal husbandry skills. Griffin cavalry, or hippogriff which would be faster and cheaper to assemble, should realistically be fairly standard in any larger country. Anything with a breath weapon becomes a prime candidate for a breeding program as well.
All of these things would not be overly costly to maintain within a kingdom. Hrm, self replicating, intelligent war machines. Every warlord's dream.