Here are some tips that might help:

In my experience with Bactria you have to make good use of your BGs early on.

I find that field battles are the way to go with the early attacks from AS.

Using forts at the river passing near Bactria can also buy you some time, especially if you try to expand towards far away cities of India.

I tend to use horse archers and BGs early on, hence field battles being most effective for my early forces.

The archer-spearmen are also very effective if used right.

After you expand a bit you can start making some nice Hellenic armies.