Whizzy's Solution
Achievement Difficulty - 2 / 5
Estimated Time Span from Fresh Start : 15 Mins - 30 Mins
As with most achievements, it is easiest to attempt this in an achievement server where there are idle players.
This should be reasonably straightforward provided you have a good line of sight and the engineer isn't wrangling. The sniper does not do very high damage against sentry guns.
Some achievement servers have sentry guns placed as part of the level. You still need to find a way to avoid their fire and destroy them.