How do I make my mob attack certain groups of mobs?

You can either make a procedure that  checks for entities of that type within a radius, and sets the entity's attack target to the nearest one; or you can just specify each individual entity as a target in the Mob AI. (Which may be quicker, depending on how many different types of entities you want it to target.) You would just add an 'attack entities of type' AI function in addition to the melee attack function.

Mindthemoods  how would you do this? does it require custom code?