In a VH/VH campaign, I was lucky enough that they gave me some breathing room, but sure enough they came. They are wired to come. Use every available trick you can. That includes

1. Build stone walls and many sphendonetai. Sally each turn until you kill enough of them, so you can take out the "heavier troops"

2. Baktria now (and especially so in the future) is river country. Some people have a grudge against river crossings, but not I. Attackers' Bottleneck is your friend.

3. If you are going to get attacked no matter what, try to find a higher elevation than the attacking army. Your missile troops will slaughter them before they ever come close.