I've taken the AGT out alone w/ a flame tank.  If GDI doesn't have advanced units nor tanks, block the door so Hotwires can't get in to repair it.  I didn't block the door once, and just as I was about to destroy the AGT, 2 Hotwires got in and repaired it to full health.I once was even able to place a nuke beacon in front of the AGT, w/ GDI destroying my flame tank w/ medium tanks.  Don't ask how, but I killed one Hotwire that tried to difuse the beacon w/ me standing right on top of it.  A few bullets to the head stopped her.    