Easiest way to win is just spamming mobile artillery. They have armor so they can steamroll most infantry, even higher armor than light tanks and almost as much armor as medium tanks, but cost 60% less in gold and oil upkeep. Artillery in this game is down right the best unit in the game cause they are the only ones that can attack 2 squares away from them in any direction including diagonally. Where as tanks and infantry can only attack 1 square away. You only need essentially 4 general skill point to get the max out of them. The Artillery Commander, Attacker, Defender, and Lucky. Also you only need to research 1 branch of the tech tree, though to be honest the game will be over even before you need to research anything since you can snowball the enemy so easily. Don't waste your money on tanks or heavy artillery, or infantry. Just spend your money on mobile artillery or regular artillery and oil production. Best general to use is the one that gives you +10% SP points so you can spam Direct Command on your 3 front line units at the start of the battle on and the reinforcements skill to heal them.
Once you get 6 units of artillery, even regular artillery without armor the game is essentially over. You can just deploy the 6 artillery units by themselves, and order them to hold ground and just pummel any incoming enemies into the dust.
Early game just buy 2 units of regular artillery with your starting 2000 gold. Deploy the 3 starting infantry units you have in front row. Place the 1 artillery you start with, and 2 artillery you just bought in the second row. Order everyone to hold ground. Use your direct command skills on the infantry to increase their battle efficiency. And spam reinforcements skill on them once they get to like 40% health. You can win pretty much against any enemy general even 4-5 stars ones like that since you break their army's morale via casualties. Keep doing that until you get 6 units of any type of artillery and you have essentially won the game at the point. You can just disband your infantry once you get 3 units of mobile artillery since they can act as your front line. I generally disband all my units and crank out nothing but mobile artillery, an army of 18 units of mobile artillery is enough to conquer the entire map. 9 units for offense, and 9 units for defense so you don't tire them out. Once you have 9 units of artillery on the field, assuming the first 3 are mobile artillery, you can just charge forward and sweep every battle.
To be efficient it is highly recommended that you "surrender" some low value areas to the enemy in Europe when they counter so the game doesn't end to quickly via victory points. Cause you auto win if you control Central Europe and 28 victory points. When playing as the UN I would recommend getting 4 date points with all the generals before getting the 5th point. Cause whichever general you reach the 5th date point on first is the one you get the ending scene for. So essentially get 4 points on everyone, and then attack Central Europe once so it orange. Then make a save. That way you can just easily fight the last Central Europe battle 4 times to get all the ending for that particular campaign.