everybody wants this so much because it will improve tactics so much. i suspect it's probably more complex to implement than it might look like though. heck unit formations (roman letters on party screen) don't even stay after a game load. much work to be done.
the idea is pretty simple, though. pick formation, F1, F12 (or whichever), type number for the enemy formation you want your picked formation to attack. it would be necessary to visualize enemy formation numbers in order to make that possible though. telling to attack unit type (instead of formation) might not be feasible or even desirable i think.