Get hot water and rinse in threw the bong and shake it around enough so that you can feel your bong being a little warm.
Once it's warm, empty out the water.
Go to home depot and get a 1 gallon container of Zep's Orange Citrus Degreaser. It is around $9 for the gallon.
Fill up the warmed bong with the Zep's all the way to the top, do not dilute it with water. You will need to find a stopper to put where the down stem usually is. Shake it for 30-60 secs and let it sit overnight. Check to see if the resin has come off, if not shake again and let it sit. It should be clean by now.