Tower Plower

I just accomplished this achievement today. Below are some confirmations and strategies regarding this achievement.

Confirmation: You must get the killing blow (as others have mentioned). You do NOT need to be near the tower. I was literally halfway across Tol Barad when I got the achievement. As of 4.1, this is not bugged.

Strategy: In order to pilot a siege engine, you must have the Veteran buff, which is obtainable by getting an Honor Kill in Tol Barad during the current attacking session. (Note that defending players cannot attack the Siege Engine due to Thick Layer of Rust, an ability protecting the Sieges from being destroyed before they are even piloted.) Get Veteran as quickly as you can so that you may pilot a Siege Engine.

Quickly drive a Siege Engine up to a tower and deploy it. Sometimes the opposing team will earnestly try to destroy your deployed Siege, other times they will completely ignore it. Regardless, go pilot a second Siege Engine and deploy it against the same tower. (Siege Engines can usually be found in pairs, and players usually deploy Sieges to the nearest tower due to how slow they drive.)

This obviously is not the only strategy, just the one I used. You may be unlucky and have your Siege Engine destroyed by the opposing team, or another player may get the "killing blow" on the tower. But don't give up hope. Keep trying and this achievement will be yours!

Good luck!
-TheHavel of Stormrage, Human Warlock

You can actually solo this achievement if you got both a Horde and an Alliance toon.

If you want to do the achievement on your Alliance char, wait til Horde controls Tol Barad. If on your Horde char, wait til Alliance controls it. If you don't care, even better.
I will explain it in case you want to get the achievement on Alliance, of course you can simply swap otherwise.

1) Horde controls Tol Barad. So Horde defends, Alliance attacks.
2) Park your Alliance char near the northern bridge at the entrance.
3) Remove your gear at your Horde char and wait til the next match starts.
4) Horde will get ported to the center. Mount up and get to the bridge to engage the Alliance NPC guards there.
5) Alt + F4 while in combat.
6) Log in with your Alliance char and kill your Horde char who should still be in battle with the guard.
7) Your Alliance char can now mount siege engines! Grab them and deploy them on one tower. The more siege engines, the faster the tower will fall.

If you are unable to see your other char in battle, remember that war mode on / off gets you to different instances of Tol Barad. This can be useful to find an instance that is controlled by the faction of your wish, too.

I think your siege must get the "killing blow".