To kill Tanks faster, throw a Molotov cocktail or a gas tank and shoot it. When the Tank is on fire, unload on him with an assault rifle.
To defeat Tanks under the Expert difficulty setting, follow these rules. Do not shoot it when you see it. Run away as you stand no chance against it in the open. Leave your teammates. If you are moving slowly due to injury, take pain pills. Do not heal; that is a guaranteed death. Shoot at it while running if you wish, but you will not be able to see where you are going and will probably end up being pinned against a wall. You should be running back the way you came. Do not worry about leaving teammates; if you are playing alone with three bots they will die anyway. Return to the safe room since not even the Tank can break the steel door. However, for some odd reason, he can still hit you -- be careful. From there, you can use any weapon desired.