This is a pretty self-explanatory achievement, but I just want to clear up one thing.
The rockets from a level 3 sentry do
not count towards this achievements for some reason.
Therefore, you must wrangle and kill a sniper with one of the following:
Mini-sentry (Gunslinger)
Level 1 Sentry (Stock PDA)
Level 2 Sentry (Stock PDA)
If you don't know what a wrangler is, or how to get one, it is an achievement item for Engineer.
https://wiki.teamfortress.com/wiki/Wrangler