Speed Metal

There are 6 [npc=69216]s right near where [npc=69326] pats. 5 are on left left side facing the cave, kill these 5 first to gain a quick 5 stacks of the buff and over 1 minute duration. Then kill the one on the otherside for another stack an refresh of duration. By then, the other 5 should start respawning. Gain your 10 stacks then start burning [npc=69326]. If the duration of [spell=136842] starts to get low, simply kill one more [npc=69216] to refresh it.

To get this achievement: you need to kill 10 electrocutioners AND BE STANDING ON THE CORPSE when they die because if you have a melee friend with you the melee will get the buff every time, however if you are both standing on the mob when he dies, both get the buff. If you are ranged and by yourself, simply run over the lightning orb left behind after mob dies to get the buff. When you have 10 stacks of the buff, kill Mo-Han.
The actual buff: Lasts only 10sec the first time you pick it up, however if you kill 2 within a 9sec time span, the buff is extended to roughly 18sec, in which time you must then have killed another electrocutioner in order to maintain the buff, and this will add more time to it as well as a 3rd stack of the debuff. The debuff greatly increases haste and damage, so killing each consecutive mob will take less and less time (be wary of this when with other players because of buff rule i mentioned earlier). At 7 stacks the buff will be lasting 2min and will not extend above this with the extra kills but kills will put the time back up to 2min. Once you get past the first 3-4 kills killing within time frame is a non-issue - they die fast.
Finding the electrocutioners: 7 can be found outside the cave next to where Mo-Han spawns, one right next to the exit of the cave on the left, which is very handy for chaining the buff so that it doesn't run out.
The first 3 you need to kill will be inside the cave to the left if you were coming into the cave from Mo-Han, though I would advise killing 4-5 to ensure you don't run out of buff before you kill next outside mob. Unfortunately the do not follow far with aggro so you cannot group pull them to the cave mouth. To save any hiccups with buff running out, i would also kill slaver which is patrolling tunnel mouth before starting to kill electrocutioners - saves time for later which is especially helpful if you have lag issues.
Killing Mo-han: Is super easy as ranged with a pet, quite easy with ranged generally, and can be painful as melee. The main ability to look out for is a cast where he throws metal (i believe it is a 0.5sec cast), when this lands it will look like an anvil with a roughly 20yard range red circle, this will cause 20k lightning dmg per second you are in it, so as soon as you see it it is wise to run out to save having to heal yourself at all during the fight, boss should follow allowing for melee dps killing. Dmg to pets is negligible and most classes with buff should take no more than 1min to kill Mo-han. Easy.

[npc=69326] patrols the western segment of The Thunder Forges, by the entrance to the Lightning-Vein Mine.

[spell=136842] is a buff given by fighting the Mogu in and around the Lightning-Vein Mine ([npc=69216]).