No Ticket

Found the easiest way ever...
Grenades Killed them so i just tried and tried and found this...
EVERY Class and Spec can do this without any spells, bombs etc needed. JUST WALKING!

https://ibb.co/vZZ3qxt

Just walk up the Line, let you wind down and repeat untill nothing spawns.
At the beginning it might looks like nothing falls down, but turn on nameplates and look at them,
you will see some fly away to the bottom after a few walks.

I didnt kill any mob except the boss... :)
just let everything fall down and have a 100% save achievement.
It can take time but they will fall down!
It took me 10 Minutes because i would make sure it works with the Pathing.
Have fun with your Achievement

A glorious reference to this legendary scene:

http://www.youtube.com/watch?v=iHSPf6x1Fdo

This achievement is done by knocking 20 adds off the side of the train during the course of the Nitrogg Thundertower encounter.

You do NOT need to knock off all 20 adds at the same time.

You'll need a knockback in your group. Some examples are:
Ele Shaman's [spell=51490]
Druid Specced into [spell=132469]
Hunter's [item=42908] and/or [spell=150381]