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.
The best way to do this achievement is to find an encounter where the enemy has no way of hurting you. This is the same technique used to train crews but is much more difficult in this case as it can only be performed late, after you have full engines.
There is, however, one ship that is perfect for this- the Rebel Flagship.
Enter the fight with fully upgraded engines, at least 6 shields (3 layers), level 3 piloting system, a teleporter and a decent boarding crew. Begin the fight by boarding and destroying the missile launcher, the ion blaster and the halberd beam. Leave the lasers (middle-left) alone.
You now have an enemy armed only with a burst laser 2. It is impossible for them to break your 3 shields, repair their guns, flee or do anything at all. All you need to do is man the engines and piloting and wait for the 5 hits in a row- you don't even need to watch it.
Best Answer
Once earned, the (more accurate) description of the achievement is: "Play an entire song on the piano with 3 players"
The criteria to get this achievement is:
Some easy steps to obtain the achievement: