S'mon said:
Soviet infantry charges vs King Tigers looks a lot like the D&D paradigm to me, with a similar result - the infantry die in droves, but in the end the KTs have to retreat or be overrun and destroyed.
Again, The issue with the Tiger tanks (or blitzkrieg, or jets, or etc.) Is that not you can't fight them with infantry, or anything else, its that they had no time to adjust to the tactics needed.. The Zulu *could* have done much better against machine guns, if they had *ever* seen one before. It isn't just the technology that is the benefit, it is the **unknown** technology that offers such a surprise that really turns the tide.
Most DnD worlds are fairly static. You know about the enemy having magic users, and firestorms, and bad assed fighters, etc. Therefore you can prepare for them.
A 20th level firestorm has a max range of 300 feet. Even a short bow has double that range. A hundred of them may give the caster some pause.
A 20th level fighter has 200HP. When the horde of 150 mooks run at himi, he just smiles. Until about 50 'random' ones stop and start reading a scroll. 50d4+50 from magic missles will sure leave a mark. (He might have a shield spell, but then you might just feint a large attack, and back up, to work through all the buffs.)
20 draw up and cast dispell magic
One of the ones in front is actually a mid level Mage with anti magic field. he looks just like a mook, don't notice until all of the fighters magic stops working.
2-3 mid level mages cast greater dispell, let the mooks clean up, and move on to another group. (imagine losing all your magic for d4 rounds, while several hundred arrows are inbound)
The 20th level cleric casts firestorm on the 500 running at him, kills 50, the other 450 run and close half the distance, kill another 50.(how many 7th level spells?) 20 charge, 380 throw javelins. Next round they 'aid other' for a grapple check.
You spread out the attack. Send 100, wait, send 100, wait, etc. (or 50 etc.) make them waste spells.
You spread out the forces, make one per 20' square, in stead of 5'. Now the area spells aren't worth much, and the fighter has to move to attack. Once you get close, you can close on the spell casters. Or just keep peppering them with javelins.
There are more tacticly inclined people here than I am, They can think of even better ways of dealing with high level people. *especially* if they are often working with that army, so lots of info about thier methods and capabilities.