hard answer
Fact is i have yet to see Axis winning in MP if SU is human player.
At the end, despite what others say, playing any axis country is far more difficult than pick an ally country (except france

).
The IC and manpower of Allies double the ones of Axis. and so, time is for Allies.
Said this, one would say that quick aggression and build rush is the way cause the only advantage Axis has is the starting position and tech.
However, if axis behaves aggressive, then US and SU will be in war earlier and so they'll have even more IC (CG going down).
Honestly, i don't know and don't know if in multiplayer axis has really any chance (unless allies players are real newbyes).
Last game i played, i took SU. I played quite shitty and not concentrated. Axis attacked me when i was eating, when i was back to comp he was already advancing towards moscow. I made many mistakes, which eneded in losing 90 divisions in encirclement battles.
Germany conquered moscow and stalingrad. But at the end, Germany lost that game too, and was losing even before Allies landing in europe.
I mean, i as SU played maybe the worst game in MP ever while German player played very smart. But SU won anyway.
It's just too easy for US and SU. Maybe the key is to take out USA very early (taking out UK is useless IMO) but it would be very hard and if you fail, you have US in war in '36 !