I agree with what the others said, just go and destroy some of their towns. I don't know how highly developed your settlements are but while you are raiding their towns you might actually want to keep one or 2 which are in a good position strategically. What I mean is a town which has strong stonewalls and which the AS would choose to attack with all their troops instead of attacking 2 or 3 of your towns. Keep some horsearchers and slingers in the town and when they besiege you attack them every turn from the town with your archers, then retreat back to town and repeat. by the time they will assault the town they are already pretty weakened.

Have a mobile army of horsearchers and some infantry to raid their towns, burn them and move on and use your horsearchers to intercept all the small stacks they send to reinforce their troops so they can only bring already damaged units to besiege you while your on the way.

That's all you can really do until you are strong enough to attack them on several places at once.