Is there any emulator I can download to speedrun the game?

Segment from rules- All emulator runs must be done on DeSmuME version 0.9.11 or newer. When submitting a run done on emulator make sure you select the “Emulator” sub-category and set the platform to “Nintendo DS” with the emulator box checked.

would the 2015 version of DeSmuME be alright to use?

The version from 2015 is 0.9.11 so yep that's fine to use.

Which emulator i can use if i play in android?

DeSmuME doesn't work on phones, so android emulators wouldn't be allowed