As much as a well played druid stands out from the others, there is no substitute for a resto druid. They have a unique set of abilities that not only allows them to keep a group of people alive, it allows them to survive even the worst training. Killing a resto druid is no easy feat, but with any luck these tips will make the task that much easier.
One on one a resto druid is not much of a threat. He will be difficult to kill, but you are more likely suited for the task. The issue with a druid is that his Rejuvenation is fast cast, while they can instantly step into travel form for a speedy escape.
A good druid is going to be doing what they can to limit your ability to do much damage to him, while being able to easily keep his teams alive. They do this by a mix of shape shifting, breaking line of sight, and even the ability to do some cc with cyclone and roots. It is difficult to keep them crowd controlled because of this, so in most cases you are going to be dealing the healer the whole fight.
Some people combat this survival by making sure to train the druid first. This strategy will highly heavy on burst damage and the ability to keep the druid in one spot. If he is able to shape shift and move, his odds of survival increase. It should be noted that during this time his team is doing everything possible to reduce your effects.
If you are a mage, a lucky polymorph may force them to use their trinket. It is not something to count on as the druid is likely going to be watching your every move and will be ready to shift or cyclone you to keep you from landing it.
Warlocks need to watch for a shift into tree form. Once they make that change you should banish them which will keep them out of the act for long enough to handle another person. It should be noted that this is another potential spot to get them to use their trinket. Removing that survival element is crucial in any fight.
Another smart option is to train one of his partners. It does help to do what you can to cc the druid, but it is more likely that you can take down the teammate than you can the druid. This will free up the druid for some easy cyclones, so ensure that your focus target is set for him and be ready with all ranged interrupts.
Sometimes the easiest way to handle a resto druid is to simply take out his friends. As healers they are not a huge threat on their own, but their presence can make a good team even better.