Buttler has been Royals' best player in the tournament so far and his 89 off 43 balls guided Royals to a four-wicket victory over Mumbai Indians in an IPL game on Saturday.
"It is challenging to bowl (to Buttler in the nets) because he has got shots all over the ground but I take that as a challenge to improve my bowling," Kulkarni told reporters after the match.
Terming Buttler as a 'special player', Kulkarni said that the Englishman's consistency has been one of the positive aspects of the team.
"Jos is a special player for our team and a knock like that, it changed the momentum of the game. And he is being doing that consistently from the last year. If you have seen he has done that for us earlier and this year also he is in a good form," he said.
The best part is that Buttler because of his range is difficult to bowl in the Powerplay overs.
"Well to international guys like Jos, its very challenging to bowl in the Powerplay overs. He is in phenomenal touch and I hope he keeps continuing that."