I just finished quite a few runs of mana tombs between lvl 65-67 with my 5 shammies. I'd say it was a fairly easy instance. The 2nd boss is a little rough, but with earth elementals, its cake.

I ran it so many times because it was great money/xp and it was fairly easy.

I'm about to ding 68 and I might just quest the rest of the way to 70.... but those instances are nice