Cast("Blunt Arrow", r => KeyboardPolling.IsKeyDown(Keys.LShiftKey), r => Core.Player.CurrentTarget),
Anytime you're holding the left shift key it'll use blunt arrow whenever it reevaluates what to cast next. You can change Keys.LShiftKey to Keys.F, Keys.4 or whatever you want. Easiest way i've found, obviously this only works with Raidbro.