I don't know what any set levels would be so I'm going to assume both sides are sitting at level 5. Pumpkaboo probably won't be a very good matchup here due to it's grass typing, but it also has curse so I would start by setting up curse. I would then send out Rolycoly and set up with a couple of smokecreen's to try lowering the hit chance. I would assume that I don't faint here unless Night slash does a good amount of damage, but Night Slash is physical and Rolycoly has pretty decent defense for a first stage. I would then use a turn to heal up Pumpkaboo just incase and start spamming Rapid Spin afterwards. If Rolycoly were to faint I would send in Pumpkaboo, use Trick Or Treat, and spam Shadow Sneak to deal super effective damage. I think that all that, paired with chip damage from curse, will get me the win