All you need to do for example is set up something like this:
Guard: (When killed) GuardFaction -100 BugFaction +100
Bug: (When killed) BugFaction -100 GuardFaction +100
Since those factions both exist on both mobs they will attack each other on site. By eliminating GuardFaction +100 from the Bug's factions only the guards will attack the bugs on site I believe. Experiment with it.
That is the easiest way to get two mobs to fight though.