If this happens, just shoot at the ground around you. That should knock him away, then you can finish him off with rockets or grenades. If you do get tranq'd, you may be screwed since the spy can then run behind you and knife you. If that happens, either shoot at the floor or prime a grenade and let it explode on you. You might as well take him with you.
Engineers are defenseless without two things.
A good SG killer is the nail grenade . . . After you kill an engineer he will most likely leave an EMP on his dead body for you, so quickly run away from his body to be safe. Enter one or more words from the game's title. All rights reserved.