I had a lot of trouble with this one as a GLD, but as a THM it seemed a lot easier. Sleep is key I think.
I stood at the side not the canyon side of the bridge and just took out the ranged guys first (putting the others to sleep)
Then picked off a couple of guys fighting my guys.
The second lot came and I took the trailing guys, think they are mages of some sort. Same idea with sleep.
I also put the dragon to sleep, then killed it when it woke up.
By this time there was only a couple of guys hand to hand with ours and I just kept blasting away with fire, transposing, lightning, back to fire when MP was full.
That was it.
The sleep management stops a lot of damage getting through to the allies and gives you chance to pick them off easily.

Never healed anybody btw