I don't know if healers worth it after tier 3. My mystic's got 4 levels of heal and he is out of spirit in a matter of 10-15 ap, while he is fully equiped with +spirit and intelligence items. His damage output is the lowest of my whole group and all these for 6 hitpoints of heal per round. I seriously think of replacing him but I find it hard to find some tier 3 mystic with the heal skill (I don't know even if it is possible, since I have this one since tier 2).
I greatly value my 2 other mystics though.
The one's got a level 2 Fountain of Health which enhances 3 of my members' vitality at the beginning of each battle. He also comes along with 4 levels of Staff Mastery and 6 levels of Inner Concentration, which give a damage output similar of that of my ranger.
The other one's got 2 levels of Blur, which lowers an enemy's dexterity in the beginning of each battle. However, he's also got 2 levels of Curse Mastery which spreads the Blur spell on 2 additional targets. Last but not least, he's got 7 levels of Staff Mastery, that bring his damage stats slightly above that of the previous mystic, without the critical hits however, making him a decent damage dealer.
I call the first one the "Buffer" and the second one the "Illusionist".
To complete the team, there's a tank with 2 levels of Taunt and 3 levels of Shield Bash (and a ton of vitality
) and a ranger with 7 levels of Critical Hit, 2 levels of Ignore Armor and 2 levels of Retaliate.