Ah yes, Stone Grove. Pretty sure it's a instant kill against anything weak to fire once you get the 3rd tier via BBI. Too bad the casting time is a bit slow.
Stone Grove/Forest/Jungle also carries with it an enormous amount of knockdown power. A well-geared Mystic Knight can chain-knockdown large monsters like (Gore)Chimeras, Cockatrices, Wyrms/Wyverns, and even Death. Slow at first, but extremely potent once you establish a rhythm.
Usually I have my Mystic Knights rocking:
Sword/Mace
Great Cannon: Self-explanatory. Arguably the most devastating skill possessed by the Mystic Knight.
Ruinous Sigil or Stone Forest: Ruinous Sigil is mostly for Daimon or Ur-Dragon busting, especially Daimon's 2nd-form or the Online Ur. Sigil combine with Great Cannons can dish out absurd amounts of high damage projectiles.
Full Moon Slash or Sky Rapture: Sky Rapture is a great way to dish out a lot of knockdown quickly, but can also serve as an air-dodge/dash for the Mystic Knight, and even help them cross gaps they normally couldn't. Full Moon Slash typically occupies this space, however, as the invincibility frames it affords help a lot in heated combat, especially any Fiend-Luring Incense fights.
Staff
Sopor: Sleep status, as noted by others here, are extremely potent against many enemies.
High Brontide: Not really the best spell, but the lightning whip effect is so fun to use.
High Frigor: Mostly for stepping platforms, but effective against a few enemies.
Magick Shield
An Elemental Trance: Being able to self Enchant, as well as buff the entire party is extremely useful. It also allows me to set magically-oriented pawns up with "leaner" spell lists, so as to avoid having them cast buffs when I would rather they be (sync) casting High Archmagicks, like Fulmination, Glicel, Seism, or Maelstrom.
An Elemental Riposte: These are really under-rated moves. Blessed Riposte is probably my favorite because the spheres of Holy light released on a Perfect Block hit multiple times, stun enemies, and can even trigger numerous Great Cannon shots. Plus, the slight frame leniency that those moves offer makes Perfect Block Canceling easier/faster, without having to burn an Augment slot on Prescience.
Abyssal Anguish: It's that damn good, even against enemies resistant to Dark. The multiple hits really add up in terms of damage and hitstun/knockdown.
As an aside, it's somewhat disappointing how three of the more interesting skills associated with the Magick Shield are so under-powered.
Demonswrath is an extremely flavorful technique and bears a strong resemblance to DMC3/DMC4's Royal Releases; you build up a meter by performing just guards, and the more attacks you perfectly block, the more potential damage is stored. While it can unleash a heap of damage, the Mystic Knight is so stacked in that department this skill is readily overlooked.
Holy Aid is also cool, converting blocked hits into healing, with Perfect Blocks restoring more health and even going beyond the "white health" that can be healed by most magical means. Like most recuperative (or buffing skills), it's utility suffers a lot because of the way items work and are balanced. Playing this game with little to no-item usage really changes the valuation of skills, abilities, and party composition.
Holy Furor is a good spell, it's just that Bitterblack Isle is filled with so many Surging Light Tomes that carry the same effect, but casts faster, and has no stamina drain drawback.
