Hey there, been getting elbow pain when am paddling, i pull my hand from an outside to inside (under board) stroke which gives me power but also very sore elbow.

anyone with advice on how to paddle better / stronger or maybe exercises to do to strengthen it ? 

Hiya mate

 I don't think there's anything wrong with you're techinique.  I do the same, and it's also mentioned in Kelly Slaters first book that he uses the same technique as well.  You probably need to do more stretches and warm ups. I've got most of mine out of the 'Yoga for Surfers' series.  Sure you look like a bit of a poof on the beach but when you getting old it's better than not surfing ;)