I'm no scripting wizard, the best I can do is script you a Pikachu, but have you tried something like...
When in the case that there's two, three, or four actors with equal threat amounts, perhaps you can generate a random variable of 1, 2, 3, and 4. Whichever variable it lands on would be the monster would attack...Of course it might turn out to be very unoptimized or so (4! = 24 [If you see no relation to this thing, then ignore this]), but bah.