Because in Pokemon Diamond you enconter him at level 70, I would say around
85～100. If you are between these levels then you are not weak enough to be defeated easily, but you can still do damage to him without knocking him out, but levels 90～100 work best I think. Since he is at level 70 you won't knock him out too easily even at level 100, as long as you don't use too powerful or super effective moves.
Remember to save before interacting with him.
Hope I helped.