The party:
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Cleric
Druid
Druid
Sorcerer

A Cleric is needed for GM Body Magic, and also concentrates fully on Dark/Light Magic once it becomes available. The Sorcerer is needed for Lloyd's Beacon, the only *really* useful elemental spell that becomes available with grandmastery. Druids become the best all-around blasters with their GM Meditation and better hit points than Sorcerers.

Minimize everyone's starting Luck, Speed and Accuracy values and Intelligence for Cleric, Personality for the Sorcerer. Boost everyone's primary casting stats and Endurance to at least 21 to get the +4 bonus, and use possible leftover points to boost Strength. Dwarves are propably the best race due to their Endurance bonuses. Do NOT take elves. Pick magic skills only, as this saves the largest amount of money... not that it matters soon but anyway.

Utilize the Day of the Gods pedestal in Emerald Isle to boost statistics to rather insane levels. Gather up money to buy the Bow skill for everyone and kill the Red Dragon using the "run-turn-shoot-turn" strategy and multiloot it to get good equipment for everyone. Look especially for stuff that boosts skill levels in various magic skills (especially Earth Magic), ID item and Alchemy. Finding a nice set of SIX daggers, ONE good mace and ONE good shield should also be high on the priority list.

Key spells and abilities:
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Cleric - Concentrate on Body Magic and raise it to Expert level ASAP to get the Regeneration spell. Get 1st promotion ASAP to gain access to Protection from Magic, and remember to GM in Body Magic right after 2nd promotion. Regeneration allows this party to disregard traps pretty much completely, just setting them all off and regenerating. Concentrate on Light/Dark magic once you get it, devoting all (or almost all) your skill points to it. Remember to develop the "Repair" skill as well, but M7 should be enough.

Sorcerer - Concentrate on Fire magic early on to get access to Fireball & Meteor Shower, and make sure you can GM Water Magic as soon as you reach your 2nd promotion. GM in Air should be a distant second priority for long-lasting Invisibility & Sparks. From there on, concentrate on Earth Magic only, disregarding even Light/Dark Magic. (Although a few levels in Dark magic could be handy for a second caster for Shrapmetal & Dragonbreath.) Remember to develop your ID item skill as well, but with a good item to boost the levels, Expert level should be plenty to ID anything in the game.

1st Druid - Concentrate on your Alchemy skill to be able to brew Haste, Divine Cure and Divine Power potions for your team. (The Haste spell is bugged, it doesn't work at all.) Use low-level blue & red reagents to brew simple potions, as you'll be running low on yellow reagents. Aim to GM it by your 2nd promotion if you're playing Dark side, but leave it at M7 if you're playing Light. Concentrate on Earth Magic as much as you can, ignore Fire & Water magic and put a little points into Air magic to use the Sparks effectively - or Master it if you wish to get access to Fly earlier, as the Sorcerer is busy with other magics for quite a while.

2nd Druid - Concentrate on Spirit Magic early on to gain access to the Heroism spell. Don't put too many skillpoints there if you plan to choose Light side, though, but a Dark team could use skill levels in excess of 15 or even more. Ignore Fire & Water magics and concentrate on Earth Magic, with a few skill levels in Air Magic for better Sparks.

Earth magic has two excellent spells - Blades and Rock Blast. You get a very high damage single-target spell and a very high damage multi-target spell with only one magic school. Acid Burst is a bit better than Blades but the other Water spells are quite lacking. Rock Blast is a bit trickier to use than Fireball, but has quite a bit better damage and a very important ability - it can bounce to reach behind corners! Neither can really match up with Shrapmetal or Dragonbreath, but those Dark Magic spells also eat up your party's spellpoints VERY fast and are quite a bit overkill in many cases. One primary Dark Magic caster is more than enough, believe me. Please note that the Earth Magic's final spell, Mass Distortion, allows for a double saving throw - the first to negate the effect COMPLETELY and the other one to reduce damage. Therefore it won't work very well against Titans, Dragons and such that have excellent saving throws against all magics.. and other monster types have less Hit Points as well.

Even if this party is rather caster-heavy, their melee damage is very high as well. With Haste potions and a high-level Heroism spell, these characters poke enemies with high damage so fast that even Minotaus and such are a piece of cake. Try to avoid ranged combat in the mid-game, especially before your mass-destruction spells (Rock Blast most notably) have developed enough to deliver massive damage.. you're most likely going to do much better damage at melee range.