Raid Rx is going to help you to quarterback your healers to win. Do not try to the latest and crucial instructions about killing of a progression boss. If you have to care for your targets, you should make sure the other healers have the right ones. Farm bosses will face challenge, especially when I forget to tell someone what to do. It is very necessary for you to ensure that the bases are covered.
The first step is to make sure enough healers for the raid boss are available. There is a holy paladin that had to step out once during a raid. Besides, there is a shadow priest in the raid that is able to switch specs and heal as he has the gear and the skills to do it. The holy paladin takes off when running back into the raid instance. At this moment another player would replace him. The third phase was hit by a few minor bumps and bruises, but nothing could be well handled. Unfortunately, the raid began to drop throughout the place after that. There had been changes to healing. The position of a holy paladin was replaced by a discipline priest. You can find the problem by looking at the DPS meters. Do not forget to tell the shadow priest to switch specs and play discipline healing.
You should first take good care of any key roles in an encounter. If an encounter is heavy on dispels, you can usually get away with one person. Things such as Infest on Lich King would use one or two dedicated players to reduce those effects. Portal healers should be decided on ahead of time, which is indispensable part leading to the success of your raid.