River lines and armoured counter punching in weak spots is critical. If you let the Germans do all of the attacking, you will lose. If your armor tech is low and or you have few armored units in a reserve line behind infantry, its all but over.

You can build many weak, low level infantry units, just to slow down the progress of the German tanks.

I find as the first reply is the way to go. Also, focus on building army level units and have them placed early on in key cities, so they benefit from digging in. Restrict at first only spending PPs on tank development. Place armour and mech in reserve in a couple of key locations ready to counterattack.

Nah, definitely not the best way. Straight away, you should upgrade two infantry into amoured armies (lvl 1) and destroy Romania, Bulgaria and Hungary (in that order). Germany will attack you with low level tanks (with how early on it is in the game) and if you upgrade your infantry into amoured armies you should be able to stop them no trouble. Next, take out Finland then march straight through Poland then Germany (while researching as you go). I did this and took Lisbon by turn 40. Good luck.