I am just theorycrafting. What if you use 4 grounding totems. That will stop his diseases. You can use 1 disease cleansing totem to make sure they stay off you. I think they can spread their diseases and do AOE damage based on diseases, but if the totem takes them off, might not do as much. They might try to burst you, so when they jump you, do a chain heal at the start, should only take 2-3 secs. If he kills you in < 3 secs, then he should kill you no matter what you do. In other words, stop trying to burst him down and try to survive. He probably deliberatly selects a char and focuses on it, so put in a lesser healing wave on 2 of your chars to heal that one. I use:
/cast [target=focus-target-target] HealSpell (doesn't matter who has aggro, it heals that person)
He will probably want to do a lot of melee damage, so even if you don't like it, I would use stoneskin and healing totems. He is going to try to crit you and parry your attacks, but I can't think of anything that would help against that, sorry. I would just focus on slowing his advance by you having more healing than he does. Think lesser armored pallies rather than more armored mages.
Good luck.