1. Contact Valentin: medium, hard 
2. Pursue Ourmurov and Natalya: easy, medium, hard 
3. Minimize civilian casualties: easy, medium, hard 

In the easy level, you simply have to reach the end in your tank. In the medium and hard levels, there is a cool, but weird extra objective, to meet with Valentin. 

To meet with Valentine, run forward and lean left around the corner. Kill the three soldiers coming your way. Don't get hit, these guys aren't that good and strafing will keep them from missing. Or simply kill one and let the rest come to you. Pick up their KF7 Soviets. Head toward the tank and to the left of the jeep is a guard who's taking shots at you. Kill him and then turn around and in the corner is another one. Kill him, too. A third and final soldier will attack from behind a gate. It's pretty easy to figure out what to do with him. 

What next? In the medium level, you must make contact with Valentin first, and then jump in the tank. In the easy level, there's no need to speak with Valentin, so jump in. For the medium and hard players, walk out of the gate and take a left down a narrow alley. Follow the alley, killing about a half dozen stray soldiers along the way. Don't get hit, you need all the health you can get. Follow the road and a wide, long street with about four soldiers will appear. Kill them and take notice of the shots coming from the far building. If you walk all the way to the end, look up close. You can see soldiers in the rooms and you can jump through a few of the windows and kill these guys if you want. Don't bother with these guys. There's a door that clearly means for you to enter into it on the building to your left (if you didn't enter the far building). Go in, take a left, and you'll see Valentin in a small room there. Talk with him. 

Valentin will extend the time you have to rescue Natalya, but only if you listen to him. Walk around Valentin so that you're facing the door. After Valentin talks to you, he'll run back to the tank. Follow the streets back to the tank and get in the tank as soon as possible. A message will flash on the screen telling you that Valentin has contacted his friends, and you will receive double the time you started off with, 5 minutes. 

In the tank, the fastest path out is as follows. Drive out the gates, turn right at the fork, and follow the street for a few blocks, turning when it does, until you reach the next intersection. Go straight and you'll hopefully run over several enemies (and hear their yelps of crushing pain). A mixture of civilians and military roam around the streets, so be careful. Civilians wear red and the enemies wears green. Switch to the Soviet occasionally because it's more effective than shooting tanks shots at everything. Troops are faster and can make better turns than you, and your cannon just can't hit them at close range. You can also turn the head of the tank as well. 

At the next intersection, take a left and blow up a few cars from a distance. Entering an explosion will cause serious loss of health. (In at least two instances, you'll need to blow them up to clear the road.) Beware of the path of blue land mines that a civilian is running around. Shoot one cannon shot into the patch and they'll all set each other off. When the explosions finish, drive through. At the next intersection, go left, then the next right. Next you'll see several troops shooting at you, so shoot them or crush them, whatever seems most cruel. You'll know when you've reached the end: There are two cars parked in front of an opening of the street. Make it there alive, and you've completed the level.