Batman: Arkham City

Where i can do the riddler physical challenge:" Hit an explosive with a batrang before it's launched etcetc"?

I've completed the main game,all side quests,399 riddler quests and this is the last! I've returned to the museum,in the gladiator pit as it described into another question board but no enemies spawn. What can i do? PLS help me!

Well, in that case, you might want to leave the museum and enter an entire different part of the city, the steel mill maybe, then return to the museum.
Or you start looking for ppl in the streets, and confront them near throwable objects like gas canisters or fire extinguishers.
If everything else fails, go to new game + and try it there.

When you fight joker in the steel mill or when you meet penguin and you have your first big group fight.

In the museum, the war room section I believe, there should be canisters that explode. If you haven't revisited that section, enemies should be there. Eventually one should pick up a canister. If that doesn't work, see previous answer for info.