It's a bit frustrating reading about how exhausted the Ash'aman become during battle, but considering how they channel, it's really no surprise at all.

Consider the hole in the wall in Maradon, a disaster according to Bashere. All the Ash'aman needed to do was weave a barricade of Air and tie it off. It's almost the simplest weave, and the first weave in the test for Aes Sedai... Moiraine shows how it can be used to contain things. Even enemy channelers would need to get closer to unravel the invisible wall. The Aes Sedai directly used this tactic at Dumai Wells to stop an army they could never have destroyed alone, and considering the Ash'aman relieved them and replaced it with a wall of their own, they actually use this tactic too.

At the same time they could have tied off walls of fire at that point, or even on the battlefield. The Ash'aman also don't use blossoms or deathgates despite being taught them by Rand. It's all regressed back to simple fireballs and lightning until they're so exhausted they can't move.

With tied off weaves a group of channelers can annihilate a non-channeling army, but this element has been ignored quite thoroughly.