If (or more likely, when) they do get in AGT range they tend to turn their turrets onto the AGT and ignore vehciles, at this stage I like to ram them with my humvee/medium tank (getting close to a FT is generally sucicide, but when it is shooting at the AGT it's quite safe to approach from the side.)

Ramming is a suprisingly effective tatic, it can throw their aim off, keep them from reaching the AGT for longer (causing them to take more hits), or possibly convince them to aim at you instead, resulting in less damage to the AGT.

If all else fails I'll even drive (or wedge) my med tank between the FT and a more expensive target (Mammoth or AGT), this prolongs the life of the target, and it's no loss if my tank is destroyed, because then I get to go C4 happy on the FT's.

Overall, I think it is fairly well balanced. What makes FT's seem overpowered is that it is easier (for a newbie) to successfully flame rush, than it is for the same newbie to defend against a flame rush.