is there an emulator with online battle system? I'd like to battle against other trainers.
I suppose you are Referring to DeSmuMeAs far as I know, no. But I do know that people (such as a friend of mine) have been able to hack and exploit a DS Emulator called No$GBA (I'm sure you've heard of it) so that the starting screen is the same as that of a normal DS. Using the right .bios files and .sav files and using the proper methods of extracting them into certain folders, I think you will be able to battle online as well.