Shouldn't be too difficult, just some work required. You'll need to make multiple effects, each a copy of the original damage effect used by the units weapon, but for each one you have to change its scan filters radius (this controls the AoE of the weapon) and make them increasingly bigger for each effect, depending on the intervals you want it to change at. Then create validators to check the # of behaviors that the source unit has of the type that increases its AoE. Add each validator to the appropriate damage effect you created. Finally create a Set effect and add all of your damage effects to it, then set your Weapon to reference the set effect. This should allow your unit to only use the appropriate damage effect depending on the number of buffs it has.