With a priest druids aren't TOO hard (this strategy works better on paladins). Basically the priest mana burns the druid dry. You (the rogue) can go on either, it doesn't matter too much, just keep the warrior off the priest. It might actually help to be on the warrior so the druid has to worry not only about his mana but also about saving his or her warrior. Druids are annoying with cheeta, innervate, lifebloom, and everything else, but if your priest and you can outlast the warrior long enough to manaburn the druid dry, you should be good. Imo feral druid + warrior are harder to kill with a priest/rogue than resto, because the druid has +healing and can just go bear or cat to avoid mana burn (not 100% sure if this works, but I'm pretty sure).
If it's possible, sap the warrior and stunlock the druid. You can keep him that way for 9 seconds (a little more with good timing / lag), and the priest can get about 3 mana burns off, which I think is 3000 mana? The druid will probably have 9-10k (keep in mind these aren't exact numbers, just basing it off of what I remember from the top of my head), so the druid now has 6-7k. Then you can vanish, resap the warrior, and stunlock all over again (assuming the warrior doesn't trinket and the druid doesn't cyclone).