Prototype Prodigy

Ridiculously easy as of 6.0. Doing this as part of Glory of the Cataclysm Hero. Just ignored the [npc=49208] and nuked boss for achievment.

Now when Mists of Pandaria has arrived this achievement is easily two-manned. Just leave the Reaper up top (don't even use it on the bossfight) and nuke the boss to dead. Reaper has been on the top of the stairs the whole time and achievement earned!

This is a guide for Prototype users

I did this my 1st time I used the [npc=49208] and it is a piece of cake if you know what its abilities are.
It has an energy bar (steam pressure) just like DK Runic power or Warrior Rage that fills up when you use the 1st ability and empties when you use the other two. The difference is that it doesn't decay when you are out of battle.

The Prototype has 3 abilities as you see them in the action bar:
1. [spell=91723]: It's your main DPS hit. It generates 20 energy every time used.
2. [spell=91726]: It's your opening move where you can charge & stun the [npc=49229]. It uses 40 energy and the important thing to know is that it can be used it at 0 distance.
3. [spell=91727]: Deals damage and knocks the elemental back. Uses 20 energy. You will never use it exept from emergencies where you can knock an elemental down from the ramp.

Strategy
The elementals will always spawn behind the steam pump and by completely ignoring you will try to go up the ramp. The most delicate part of the encounter is the 1st elemental where you don't have energy. Stand near the pump to pick it up quick and use the 1-1-2 rotation while moving together with the elemental. From the 2nd and on and as long as you have at least 40 energy use the charge first (2-1-1 rotation) and the achievement is yours as long as everything goes smooth upstairs :)

P.S. If you are quick (and risky) you can squize a 3rd Strike before the charge (2-1-1-1 rotation) and have an excess of 20 energy after every rotation.