How to earn the “I am a genius!” Achievement

Here's my suggestion, based on the actual circumstances that earned me the Specialist achievement.

Play defense on pl_badwater. Make sure your team has a few competent damage sponges (Heavies, Soldiers, and Demomen) who know how to retreat (you may need to explain that bullets are hazardous to your health).

You want to stay alive. You can facilitate this by always sticking to a buddy. The best buddies to stick to are the ones that hang around sharp corners near narrow corridors:

In these locations, you and your buddy can stay behind cover in the yellow area, and once they are fully healed (to 150%, conditions permitting), they can venture into the red zone for ideal damage exchange along the highlighted lines of sight (where Heavies, Soldiers, and Demomen have a captive audience, so to speak, of the cart's escorts).

Note that the general advice here (be on a good team, play against a bad team, heal everybody) still applies, and that you'll still need classes like Engineers (who can help by putting dispensers near you) to cover your back.

Scrakes are the enemies that chase you around with a chainsaw.

I haven't had a chance to play Killing Floor yet since the update, but if it's anything like the Christmas update, Circus Scrakes have replaced regular Scrakes in every level.

According to the KF Wiki:

For the Summer Sideshow he was reskinned into "the Incredible man monkey", a giant cymbal monkey, this time retaining the chainsaw arm.

I believe they start appearing in the fifth wave during a Long game, third during a Medium game.

The Crossbow is sold by the in-game vendor for £800.