How do you beat dusknoir at the end of hidden land?

You need to have your team leveled up to at least level 40. Make sure you have lots of Reviver Seeds and Sleep Seeds. If you can keep him asleep the battle shouldn't be to hard. Also having a move that attacks from a distance will help. Since you also have to battle Sableye you should try to have a move that will hit all your enemies.

Use a x eye seed on dusknoir and have alot of reviver seeds and be at least on the level of 40 or higher and have alot of oran berries as well.

You ,your partner or both should be lvs between
40{min} or 70 higher would help.
Take my advice it worked for me.
You or your partner should know dark type moves
Dealing with dusknoir and or sableye
Good luck!

Lots of Oran berries, reviver seeds, x eye seeds and sleep seeds. Also have dark type moves and/or moves that can hit multiple enemies at one like vacuum cut and also have moves like flame thrower so you can hit the sableye/dusknoir from far away