Here's how to do it:
Now in PTR Resto Druid overhealing from rejuvenation stacked into mushrooms to increase healing done, however, once you have bloomed a fully stacked mushroom in resto you then swap to boomkin, pop cds and explode 3 mushrooms under a player, the healing from the original mushrooms is converted to dmg ~130k depending on HP doing a total of 300-400k dmg with 3 mushrooms up, so to sum it up;
1. Spam Rejuv in Resto until Mushrooms cannot absorb anymore healing
2. Explode Mushrooms in Resto
3. Swap to Boomkin Spec
4. Place Mushrooms under your target
5. Explode
6. ????
7. Profit
It also stack double in resto, so if you bloom your mushrooms then stack them again the healing doubles (upto about 700k healing from 3 mushrooms)
EDIT: I'm pvp power stacked so if you aren't it will do considerably less dmg