If it's XP, then the screen tinting will happen after the entire attack happens, which I gather isn't what BEWD wants.
Let's see. An easy solution: Common event: darken screen, show animation (all enemies), inflict damage (all enemies), wait, lighten screen again.
Or you could do some variation like CE: Darken screen, force action (immediate), wait, lighten screen. But if you do that, make sure that the action that you force isn't one that calls the same common event. You could always create the "Boom" part of the skill and have the CE teach the caster that skill, make them use it with Force Action, and then immediately forget it, if you liked.