Im more afraid of AA Truck than M20. You can faust M20 and finish it with MG.
Fausting AA is almost impossible and you need well placed MG with vet or Pak (to keep it at distance) and double pak for instakill.
The satisfaction when you bait m20 or AAHT into teller mines and suceed, is probably the best feeling you can get in the game vs USF. But then, of course, you get overwhelmed by rifleman and "free" elite units. Which kills it.