A Noob's Guide to TF2: Engineer
Part One: How to get a pipe bomb to the face
1. Guard the intel room on Convoy as an Engineer.
2. Wait until an enemy Demoman crosses over onto your intel room's roof.
3. Demoman will then drop down to the convoy's windshield and taunt.
4. VERY IMPORTANT: Make sure you are standing on the table, and taunt back.
5. While you are immobile from taunting, Demoman should then run up to the opening on the roof and deploy pipe bombs as quickly as possible. Just four will do it.
If you are able to follow these steps and successfully achieve a pipe bomb to the face, sending you back to respawn and leaving your sentry vulnerable to the assaulting Demoman, then congratulations. You are as bright as I am.