As stated, I'm doing a MDP, with all six flavors: Music, Engineering, Divinity, Alchemy, Psionics, and Wizardry.

It seems that Instruments and Gadgets cover all six realms, with some decent offensive and defensive stamina-cast items.

The Priest is heavily slanted towards the Divine realm.

The Psionic is heavily slanted towards the Mental realm.

The Alchemist is slanted towards the elements (Fire, Water, Air, and Earth), while having absolutely no Mental realm, and a weak Divine realm.

The Mage is the most balanced, having decent spells in all realms.

Of course, you could argue that a more "effective" party would consist of multiple Fighters, or Mages, etc, but I prefer variety in my parties, with the limitation of only one profession in each party (except Bishops, who can do multiple spellbooks).

Aside from the Mental realm, where they get no multi-target damage, and the only single target damage spell they get is 7th level, so it doesn't appear until very late (and you'll probably want to prioritize the nuke instead). Except that, from an MDP perspective, Priests don't get any multi-target damage spells in this realm (and damage is what you're most concerned about when you're looking at spells to cast in an MDP).

(You are using lots of direct damage spells, right? If not, why are you playing an MDP?)

Basically, you will want an attack spell to train each realm. For example, the Mage can train her Fire realm early on with Energy Blast, until she graduates to Fireball, then Nuclear Blast.

I have found that, for example, above water (Bayjin and Sea Caves come to mind), magic can be layered effectively with this party. The Priest casts Lightning, the Alchemist casts Fire Bomb, the Psionic casts Psionic Fire, the Mage casts Fireball. This combination takes out those Rynjin and crabs that are weak to Fire pretty effectively and cheaply (except Lightning is expensive, SP wise).

Also, against those high-HP, low level monsters (Rapax, Hogars, and Swallowers come to mind), you're probably better off going for instant kills (The Priest has Death Wish and Instant Death, the Alchemist has Quicksand and Death Cloud, the Mage has Asphyxiation, and the Psionic only has Instant Death (though he does have Insanity and Psionic Blast to mess with any survivors).

Conversely, against low-HP insects in the Swamp, for example, you're probably better off with the guaranteed damage of AOE.

This is why heavy Monastery magic training.