The people you are meeting in DF probably don't know the fight that well and are dying to silly things they would easily avoid if they knew what to do. The fight itself is pretty simple if you know what to do, but would seem chaotic if you didn't. Healers can also be new, but they have the benefit of healing themselves immediately if they mess up... so healers are much less likely to die, but it does still happen.

Keep in mind that tanks should be using cooldowns at moments when they know healers will be busy (even without people messing up), such as when magiteks are out and the meteors are about to drop. It's not so much that healers are too busy healing others at this point, but they are forced to dodge the magitek beams while healing the tanks, which can make them a little delayed if they are avoiding as they should be.

Tanks, healers, and DPS should all have Ultima on focus target. Knowing when he's going to purge or pushback is very helpful.