(adsbygoogle = window.adsbygoogle || []).push({}); Not seen this anywhere so I'm sorry if it's known.

Stick your sentry down and let your partner deflect the bullets.
When the sentry stops to cool down stand in front of it but your partner stays where he is.
When it starts firing you will get the points as it's trying to shoot through you but hitting your shield.
When it stops again just move out of the way and your partners shield has cooled down so will get full xp again.

Have fun.