Is there a way to have a computer attack a particular unit in a group?
Ex. There's a group of marines and one medic. Can you use some sort of combination of triggers to have a computer attack the medic and not the marines?
None.

We can't explain the universe, just describe it; and we don't know whether our theories are true, we just know they're not wrong. >Harald Lesch
The only way is to give the marines to a player allied to the attacker.
Attack priorities are hardcoded.