could maybe use a dummy weapon with weapon impact ability that spawns an invisible entity which applies weakening effect applied to target squad in radius but with apply_to_squad_type for effected squads (e.g. eldar_banshee_squad, necron_pariah_squad, something_else_squad, etc) and in theory that could probably work reasonably well since it would have no effect on incorrect targets
Edit: (could still be a problem if the same target is underfire from two such dummies, (could maybe make modifier exclusive though ?) (or make it work backwards and boost a targeted squad who prefers you as an enemy slightly with exclusive modifier ?))