Russia 1939

No extra general needed actually, remember to multi-task Nazi Germany and Imperial Japan, control them before they expand, then they won't have resources to advance. And upgrade air level before doing it OwO

I think you're at the wrong stage to do it. Just focus on earning the one-time medal prizes for completing conquests. You don't have to finish under 65 turns.

The medals from those conquests plus a bit from the campaign (be sure to not fall short of 3 stars each) should be enough to earn yourself 1,200 medals for Badoglio. He's the best general for the beginner's stage of progress in WC3.

Japan is a lot easier to finish under 65 than Russia in 1939. I would suggest practice, and don't waste too much time dragging through the central Asia. It's empty. The small city Yaktenburg (you know which one) is easier to capture from Moscow than all the way from Mongolia.

The main path of expansion should go through both America then to Europe, and through India to Iran to Turkey. Middle East do offer some free cities (be sure to capture without lingering too long) that allows you to enter the Southeast Europe and Africa from that middle east region.

But honestly, finishing a conquest under 65... that's a goal a bit down the road. It's more important to complete conquests under 300 turns to reap the benefits and bulid up the troop techs.

Aim for air tech, but tank, artillery, and infantry techs do help too. Air works best with a technology level of 5-6 or better. Tank, art, inf all work better at a level of 3-4 or better.
(Don't ever upgrade the bunker tech, bad idea)