There were way, way too many layers on the defender side. The ponies barely got to move before stoppin' for another round.
Archers get damage bonuses against HB units (which is why they tend to bring in high death counts regardless of victory or defeat) so havin' the HBs stop literally every turn to fight a rainbow let far too much damage get on them before they got close. They never even made it as far as the b'listers.

No real damage came from the ATs; it was all the archers.

That goes down to my last section; the defence was layered up better than your ponies. The biggest problem w/ ponies is that they require proper cleanin' before their attack or this type of thing happens. If you knock three or four of the defendin' layers away, suddenly all those archers will die.

Although w/ that example, it's crazy easy to clean. A few warrior bombs and maybe even an archerattack (30k or so) will annihilate all those layers.
Ponies attack after that and go straight through to the b'listers, all the archers die and you take no more than 40k losses.