This problem has different solutions depending on how you want to play.
If you're just looking for the achievement, one solution to your problem is to do whatever missions you want, choosing the side of good, until you get relatively close to your level point. Then go into a town and steal (for minor karmic reduction) or lay waste to entire towns (for major karmic reduction). You can quickly lower your karma this way. Then, once you're neutral, go kill some random creatures until you level.
(If you're looking to steal, a good place to do it is at Dukov's place… he's got stuff everywhere.)
The "nice" thing about this system is that if you stay on the path of "good" the whole time, you can do this at each achievement point: be bad until you get to the right karmic level, level up, get your achievement, and then restore a save to get back to your good self. You can quickly get "good", "neutral", and "evil" achievements for all levels on one playthrough this way.
(I did this, because I don't really enjoy long-term playing as an evil character… but I do like my achievements!)
You could also do this as an evil or neutral character, but it's much easier to quickly drop from "good" down to your new goal, than to work your way up.
If you find that you're close to leveling, and you're at the right karmic level, and don't want to take a chance on changing your Karma by doing a story-type adventure, you can always go wandering looking for random bad people to take out. Going near the Washington Mall can get you some XP fairly quickly, and you can go inside the capitol building and clear out large areas without picking up any Karma, positive or negative.
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
Best Answer
Not sure why it wasn't on my map before, but today when I played I did in fact find the Stay Frosty,Big Chill and Mercy Kill missions (in order) on my map and was able to play them, so you can play them after the end of the game, though playing certain Evil sidemissions on the second island (Floodtown and Gas Works) will lock out the missions forever (as my Evil playthrough confirms).