I think you are seriously underestimating the Avatar crowd. If they were willing to kill and use their abilities correctly, they could easily win.

Toph just Metal Bends their armor to crush them. Or their guns. Or opens the ground up under them and closes it again, then proceeds to crush them.

Aang just bends up a tornado and lifts all the Smurfs into the air, where he proceeds to fling them several thousand feet up.

Katara blood bends the lot of them, making them shoot each other. Or just bursts their brains. Or hearts.

Not sure what Zuko can do (I haven't seen the second half of the third season so he might have learned how to create lighting, in which case he may be relevant).

Aang could also go with blades of air, super thin and sped up to extremely fast speeds to slice up the Smurfs. Katara could do the same thing with water.

Toph could make the ground the Smurfs are on all the sudden shoot up several hundred feet.