How can I achieve Patent Protection?

The Engineer achievement Patent Protection is eluding me.

The achievement's description is:

Destroy an enemy Engineer's sentry gun with a sentry under control of your Wrangler.

I've destroyed mini-sentries and regular sentries currently being built with a wrangled Mini-Sentry, but still haven't gotten this achievement.

Are there any restrictions as to what type of sentry you're using, what type of sentry they have, and the enemy sentry's build status?

Solution 1:

You will not get the achievement if the enemy sentry is in the construction process and it must be a regular sentry. Your sentry on the other hand can be any kind.

The Wrangler allows your Sentry Gun to shoot at an enemy Sentry Gun from outside of its range, unless the enemy Engineer is using the Wrangler as well. On the offense, try to haul your Sentry Gun into a position where you can easily attack the enemy Engineer's, the element of surprise is often enough to force other players to abandon their building nests. Unless your Sentry Gun is level 3, try to make sure the enemy Engineer is not nearby, or kill him or her before attempting to destroy the Sentry Gun, as the Engineer will be able to repair any damage done to it otherwise. Be on the lookout for other Engineers trying to deploy their Sentry Guns around corners and use the rockets of a level 3 to destroy them. Destroying a Combat Mini-Sentry does not count towards this achievement.

http://wiki.teamfortress.com/wiki/Obtaining_Engineer_achievements