Sample Mystic build and progression:
Mystic 1 - HP 8
STR 8 - DEX 12 - CON 10 - INT 14 - WIS 15 - CHA 13. Feats: Healing Devotion,
Magical Training (cast as sorcerer, Prestidigitation, Message, Launch Item), Precocious Apprentice (Zone of Truth).
Flaw: Noncombatant
Non-human: No Diplomacy at level 1, then no Knowledge (arcana). Gain Flaw: Inattentive.
Skills: Max Concentration, Diplomacy, Knowledge (arcana), Knowledge (Religion), Spellcraft.
Special: Sun Domain: gain ability to turn undead as a Cleric.
Spells known: Detect Magic, Guidance, Create Water, Read Magic, Conviction, Resurgence
Advancement:
2nd: Learn Mending.
3rd: Enter Church Inquisitor Prestige class. Use extra skill points for Sense Motive and if non-human Diplomacy, otherwise Bluff. Gain the Inquisition domain (learn Detect Chaos, get +4 to Dispel). Learn Protection from Evil.
4th: +1 Wis. Get Heat Metal from Sun domain and Zone of Truth from Inquisition domain. Learn Substitute Domain and Purify Food and Drink. Retrain Practised Spellcaster into Expanded Arcana (Zone of Truth - to be able to Substitute away the Inquisition domain if needed).
5th: Learn Command, Divine Insight. Retrain Magical Training into Expanded Arcana (Enthrall).
6th: Take the Divine Defiance feat. Learn Dispel Magic and get Searing Light and Detect Thoughts from Domains). Retrain Expanded Arcana (Zone of Truth) into Expanded Arcana (Zone of Truth, Dark Way).
Combat gear: Reinforced Segmented Fullplate, Extreme Shield, Razor-Sharp Thinblade, two javelins, razorsharp dagger. Misc gear.
tl;dr: At level 1 they have decent healing through turn-fuelled Healing Devotion, and 1/day Zone of Truth. At level 4 they get excellent flexibility thanks to Substitute domain (which they can end whenever they want if they want their actual Domains back). At level 6 Divine Defiance makes them very useful in large land battles: they can spontaneously dispel enemy spells as a standard action!
They also have some social skills thank to Church Inquisitor and Divine Insight (so they could be decent as lawmen or Inquisition goons).
Sadly they have no save-or-die spells (that would require switching to the Dream domain to get sleep or something, which they can't do before level 4) but they can always just buff themselves and hit things in melee anyway.
Thanks to Healing Devotion which auto-activates when they fall to 0HP or less, they're our toughest magelings by far!