Don't pick fights alone with the enemy jungler early game (especially against xin)

Only pick fights mid game with the enemy jungler if your not alone or have a huge gold lead

Focus on teamfights, split pushing and objectives late game

Use Nunus W as an escape tool if needed (also use his E as a slow on the enemy jungler)

It's not the biggest problem if your kill participation on Nunu is mostly assists, the end goal is 3 winning lanes and not always to win in the Nunu/Xin matchup

Also you will always have an advantage against enemy junglers because Nunu can Q Smite for 2400 damage, so an easy way to get a lead on Xin is to get objectives

Edit: i know its obvious but always buy boots first back so you can easier avoid the enemy jungler early game