That's an excellent question. I know it's an excellent question because I can't think of a clear, simple answer.

You shoot what's on the flanks or what's moving toward you, usually. A unit's position decides what you shoot at more often than unit type. Still, what's the best target when you do have a choice?

I prefer to shoot armored units. You get fewer kills per shot, but you get enough kills as long as at least one of the firing units is on the flank. This takes away the target's shield bonus. Peasants and other unarmored rabble are not a threat.

My favorite thing is to have some good, powerful HA behind armored units that are stuck in melee, shooting into their backs and ready to close the trap once the armored units rout.

I also love to shoot up camels, who often won't chase HA because HA are almost twice as fast. They're big, vulnerable targets.