Ramp using low reps to a top load
Then do 2x 12+ rep drop sets...works a treat.
Frequency is pretty important I found; traps need a bit of frequency (hence why it's probably better to pair them with back or shoulders than deadlifts which may be done less frequently).
IMO, getting to a true 12 rep max on a heavy lift like shrugs is almost impossible without ramping.
BTW, Smith machine is great for shrugs.