Currently my build order is the folowing:
Starting IS > IS > IS > IS > Squad Upgrade > T2 > AT Gun > Sappers > Weapon Upgrade > T3 > Firefly
Important here is to plant a lot of mines as you have to damage their engine to allow the AT gun to finish them.
The key here is unit preservation. Upgraded Vet 3 Infantry Sections are monsters and rip apart all Axis inf if you manage to build good sandbag positions. Also with 4 vetted IS you don't need any medium Anti Infantry tank like Cromwells or Centaurs.
