This is what I did:
1. Used the soldiers with knives to silently kill the sentries (the ones that patrol outside the wall), and then hide the bodies. You can pick a knife from the ground after you kill someone with it, use TAB to highlight it.
2. Separated my guys into 3 groups and took positions in front of the entrances, and explained to them that real men don't use stealth
.
3. Attacked the entrance guards, killed them, and assumed defensive positions in front of the entrances, maned the MGs where possible. After that I stayed in cover, while the whole base flocked to entrances, where they died swiftly to my guys firing from cover. (Shoot those light reflectors also.)
4. After this, the base was almost empty, except some enemies hiding behind buildings, and the guards around the main building.
5.
Now, I took my sweet time to mine all the three entrances, using all the mines from the truck. I also brought the MGs inside the wall. Don't attack the main building until you're done.
6. After defenses were complete, I attacked the main building, captured the admiral, left one guy to play sentry, while the others ran back to defend the wall.
My initial defense at each entrance was: one guy at the MG, two more ducking behind the lower parts of the wall. When you get reinforcements, notice that some of them have AT rifles. Keep these behind the defense points, in case an armor vehicle manages to get past the minefield, and the wall.
You can also take a few armor carriers during the attack, just clear them out with MG fire, then load your men in them. If the enemy tanks start firing from the distance, pull back a bit, they will either charge (hopefully, onto the minefield), or let your AT guys take care of them.
The toughest part is when they attack you with tanks, but you have AT rifles, AT grenades, and mines, so you should be able to manage it. As you're getting more of those resistance guys, keep a few of them with AT grenades in cover behind the buildings near the entrances, to give some depth to your defense.
EDIT: This was on normal difficulty (or is it called medium?).
EDIT 2: I hope I didn't forget anything, and that you'll find this helpful.