+ATK -DEF Bartre worth investing

That's a decent IV, and it even preserves his HP for panic ploy. I would easily recommend -res for him any day over -def because he can take multiple hits to his def, especially if you don't use fury.

Since he's 3*/4* available I would probably wait to promote until you find +atk -res or whichever IV you want, unless there's a reason you want to use him ASAP.