There are many ways you can do this. This is the video of how I do it. I don't know how helpful it'll be, but it's a good way to see what directions you should take.
If you're going to do it this way, use the tier II Japanese Ha-Go with standard gun.
The four levels you'll want to do are Standard Battle, Assault (Defense), Encounter Battle, and Battle for Mines. I apologize if my skill comes across as a little noobish in the video, but I swear I did much better when I did it for real. For the most part, you don't really have to do much. The AI allies are way more reliable than random people online, so you can count on them to take out most of the enemies. Also, some levels say you can win by capturing the base, but I found it a lot easier to just go for the domination victory.Standard Battle
For this one, you'll want to go straight up the center. WARNING! There will be a tank waiting for you with his cannon pointed at your face. Luckily, there's a nice little hill you have to go up, so fire a shot at him and immediately dip down below the crest until you reload. Once he is dead, push through. After the village, there will be a huge rock formation in front of you. You'll want to go around to the left and take out an opposition. You'll end up behind a few enemies. Take out what you can and find cover. The remaining enemies will come in from the left and right, so stay hidden as best as possible until they're close enough to kill.Assault (Defense)
This was the easiest of the four. The enemies always come in in 1s and 2s, so it's easy to pick them off. There are tons of opportunities for cover, so make use of it. Really, you should just hang back and let your allies spot the enemy, and only come forward when needed.Encounter Battle
It's like this in every level, but not as obvious as this one. The enemies all have a predetermined path of travel. In this case, it's go towards the center. Use that to your advantage! Especially with the tank destroyers. You can sit along the walls in the courtyards, and when they enter, they'll drive right past you. This gives you a perfect opportunity to hit their weaker sides and rear. You can also use the roads that circle, or square I guess, the courtyard to position yourself behind tanks in the center.Battle for Mines
This one isn't too bad. Head up the center and take out the opposition. Before you get to the clearing at the end of the path, duck behind the rocks and pick off what you can (you'll probably need the video to see what I'm talking about). When the coast is clear, push on to the base. As soon as you enter the circle, the next wave will spawn. Don't stop. Go right through the base and hide behind the rocks on the left. An R. Otsu will be waiting in the water. Take him out, and then use the rocks for cover to take out the remaining tanks in the distance.Edit:
Two people have confirmed that you just need to complete 4 missions, regardless on which level. So if you want, you can do the same level 4 times and it should still unlock.