Remember that you don't need to destroy FOBs, if you can capture them with a couple of infantry units those infantry are almost immortal until the supplies run out. Instead kill command vehicles in reinforcement sectors - that way the opponent HAS to respond with a significant shift of their forces.
T-55 and T-34 rushes are hard to counter because each vehicle costs only 15 points. However, they generally lack machineguns (so infantry will win) and can't keep up with the ROF of autocannons like those on the Marder I. You really can't afford to commit an equivalent points value of AMX-32 or Patton RISE tanks to that sort of fight as they'll be in your rear armour before you can kill them all.
Don't forget to leave at least two AA and two inf units to guard your base, especially if you have an arty heavy build. A zippo or T55-PzkmpfFlm can really help clear up any infantry infestations. As do sappers/combat engineers. If there is a forested reinforcement point CAPTURE IT AND STICK FLAMERS EVERYWHERE, the enemy will be unable to shift you except through massing several hundred points of artillery.
I'm terrible, though.