My party is an all-human quartet with a priest, mage, psionic, and alchemist. They are all at level seven in the lower monastery. As I progress it seems to become more difficult to kill the higher level monsters like metallic and emerald slimes. Melee is rather weak, but I can usually hold my own until magic is exhausted.
Any advice would be appreciated, as I'm not sure which direction to take with this group.
Monsters become more resistant to magic as the game progresses. Once you get to Ascension Peak, the only spells that really work are protection spells - most offensive spells are almost useless.1) Someone has run a party like yours - I haven't. So my opinions are only opinions. I never run a pure-anything party.
2) When you reach Arnika, you can pick up some muscle NPCs. You will need lockpicking ability or a very high knock-knock skill.
3) Switch one spellcaster to samurai, ninja, monk, lord, valkyrie, fighter or rogue to gain some physical killing power. Then switch one spellcaster to bishop so that all 4 spellbooks remain covered. Eventually you will be able to buy robes that will provide quick recharge for the spellcasters, and helmets that provide quicker recharge for lords, valkyries and rangers (samurais too?).
I've run a 4-caster-party.Offensive spell's do not become useless at AP (on normal; you won't use fireball, noxious fumes etc. anymore, though), but their use is diminished. I've used Mind Flay, Tsunami, Earthquake, Crush, Falling Stars, Might to Magic and Psionic Blast (because it's the only Group damage spell) until the end. Instant death spells are pretty useless (unless you face tons of low level monsters, or they're especially weak against the respective element), Death Bomb is an exception.
You will have high power cast for everyone, including the priest, won't you?
Give you Alchemist & Priest a Shield and put them in the front. Give your priest a mace, and your alchemist a dagger (I tried wand, finished the game with winter wand, but, looking back, would have much preferred a stilletto or the Canezou Dagger). You'll need the shields for defence. Psionic and Mage should stay in the back, with staffs (Staff of Doom, Flame Staff, Ebon Staff are all candidates).
As for stats:
Everybody: Intelligence
Mage: Speed (my element shield caster)
Psionic: Senses (soul shield caster; artifacts/mythology/communication champion)
Priest: Strength, Vitality
Alchemist: Dexterity (Refextion)
Worked out well.