hello, i accidentally kill darkrai by having my trace gardevoir making him sleep and damaging him using his abilities bad dreams, and now im stuck with many different ways of recatching darkrai again, i just wanna know the legitimate way of getting it back, im kinda desperate cuz ive been grinding for 3 days for it, please help me

You have to repeat the quest. To do so, go back to the dark realm and for that you need to first defeat the Horon boss north of necropolis in the dark valley, make sure to beat him without one of your pokemon fainting. Then you will be able to re-enter alucards castle where you can access the dark realm again.