So let's say that all the devil fruit users are downed by seawater. Leaving only the non-df users to win the battle.
Zoro is definitely the largest problem for team Avatar. Pre-timeskip he can already cut steel and project slashes so he can fight from a distance with lethal force. He's FTE and has immense durability. He's kind of strong
Sanji is dangerous, but pre-timeskip he can't do his walk skills to fly or run on water which would have given him a mobility advantage to fight Aang in the air.
Usopp pre-time skip is still just a sniper so he'll provide cover fire for the crew with the occasional explosive.
Franky pre-timeskip doesn't have most of his laser attacks and instead just has his more melee skills. As long as he avoids Toph he could take down the other Aang team members.
Nami would be pretty similar to a bender as far as the opponents are concerned. She could create lightning traps and try to electrocute the enemy.
So if we assume Katara is busy laying down seawater to try and hold off the DF users then the remaining OP members just have to kill her. They know their crewmates are weak and would recognize the source of the problem. They can use the coup-de-burst of the Thousand Sunny to crash into shore and fight them on the docks. If Zoro and Sanji go after Katara while Nami, Usopp and Franky fire on anyone in their way they should be able to take her out. Nothing Toph can put up can't be destroyed by Zoro or Sanji at this point so she won't be able to stop them. Aang can get in their way for a bit but he can't defeat Zoro.
Once they take out Katara long enough to stop the water assault on the DF users then it's game over. Robin can assassinate all the avatar members similar to a blood bender by sprouting hands all over their bodies and choking and restraining them. Luffy is FTE pre-timeskip and can engage in medium long range combat with devastating moves. He could drop a giant hand on any member of team Avatar he wants to take out.
