When you first hit 70 I would suggest summoning all of your earth elementals at once. If you kill the boss and all 5 are still standing then next time you try it reduce it by 1 or 2 earth elementals. They can tank fairly well. What I end up doing on some hard hitting boss fights is having one character spam a down ranked lesser healing wave on the target of the boss. The toons spamming these heals will run out of mana quickly but this should buy you enough time to kill the boss. I do that on the following bosses.

2nd boss in Mana Tombs
2nd/3rd boss in Slave Pens
2nd boss in Underbog

Good luck with your instances.