Naruto: Ultimate Ninja Storm

So to do this simply when you finish a combo that knocks the opponent up into the air (if you have support characters), you will be prompted to press ‘RB’ or ‘LB’ for a chance for one of your support characters to knock down the opponent.
Simply this must be the final blow on an enemy.
I just put in a second controller and it took me half a dozen attempts

Wasn't really figuring out what to do here based on the other 2 guides. Decided to write one with what I found worked best and also helped learn how to launch the opponent.

1) Freeplay is easier with a second controller
2) Use Sasuke - Have Lee and Kiba as your support
3) Build up a full storm guage (the little fires next to your health bar)
4) Whittle down the opponents health to about 66% of their final bar
5) Use the Flying Fire Dust Combo (Just hit B and hold either left or right on the LS during the combo)
6) This should launch your opponent and give the prompt for the support move

NOTE: Don't do this along the edges of the map. It tends to not give the prompt if you're on the edges of the level.

This is easily done during the story. I played as Naruto and used Rock Lee as a support. Weaken the opponent and when their health is low, call him in for the assist and if his flurry finishes the opponent you'll have your achievement.