You should be able to use a trigger in the manner you require. Put a trigger in the area and make it activated by east (or whichever side the spawned guys are on). Then in the on activation field put:
this setbehavior "combat"; this setcombatmode "red"
(Double check my syntax for these commands).
Make sure the trigger is set to be activate repeatedly or else it will only work on the first east guy that enters its area (oh, make sure you make its area large enough for the battle area your require.
Hope that helps.