Add the Pathfinding behavior to all of the monsters, and in your eventsheet, add the following code:
System >> On start of layout <Add another condition> System >> Pick random MosterB instance | MonsterA >> Pathfinding >> Find path to MonsterB
MonsterA >> On path found | MonsterA >> Move along path
MonsterA >> On collision with MonsterB | MonsterA >> Stop moving along path <Add another action> MonsterA >> Set animation to "atk" (play from beginning)
Now make the same events again, but switch all of the MonsterA references with MonsterB and all MonsterB references with MonsterA.
Does that help?