Start a new game in "Dead Air" under the Easy difficulty and playthe finale. Once you have made it to the tanker, restock on ammunition, grab a pipe bomb or Molotov cocktail, then answer the radio and start the gas pump. Once the pump is activated, run around to the right-hand side of the rescue plane and get ready to fight off the horde. A benefit of this position is that the horde cannot surprise you from behind. Once it's time for the Tank to appear, it will usually spawn on the far side of the plane. Run up to opposite side, then run back and forth without coming around the plane to your side. Your AI teammates will not shoot the Tank when it is on the other side of the plane. By crouching you can easily aim at the lower body from beneath the plane and take it down quickly.
The open area at the end of "Death Toll" is an excellent location to unlock this achievement. There is enough room here to fight off the horde yourself and deal with the tank.
Start any level,except for the first chapter of any campaign, and have the difficulty set to Expert. When you start the level, kill the bots. Once you do, vote to change the difficulty to Easy. Note: In Single Player mode you will automatically win the vote, but it is best to perform this in Offline Co-op mode so that you have someone to help dispose of Hunters and Smokers. Find a Tank and kill him without help to earn the achievement.
Go to the Finale level of the No Mercy campaign in Online mode. Once the helicopter arrives, tell the other players to go somewhere else and keep the other Infected busy. Go to the helicopter and get inside. If the Tank spawns near you, it will chase you. Since the helicopter is so small, it cannot get inside and will be stuck trying to reach you. Kill it and you will unlock the achievement because your teammates were too busy doing something else.