As a pryo, kill 50 engineer buildings. Any kill counts, you can use your axe, and even kill unfinished buildings.
Teleporters and dispensers are easier to kill than turrets. Most engineers group all their buildings together, so if you find one building, look for more nearby. If you find an engineer using a mini-turret/revenge crits build, kill his mini sentry, and let him crit kill you over and over again.