I see. Well, as I've mentioned, after you get all the necessary items from the neko shop, I suggest switching to Vampire for that quickcast control, which goes nicely with Purple Rain.
The four nekos, in order to cover every element.
Pyromancer Bloodmage and Generalist's Robes for high spell damage when facing glass cannons or farming.
Whispering Raiment for the initiative bonus (assuming you're at least level five assassin).
You should probably set the no-drop to earth, as that lacks anything exceptional.
Big Dictionary or Winds of Change as your wind tome for spell compression.
Healing Branch as both a water tome and a healing spell.
Lightning Rod as an energy tome.
Paladin's Tradition as a light tome.
Get Glacial Short Staff when you reach level 85. Until then (and after you outlevel it), use Magical French Vanilla Ice Katana from the Rare GGBs if you can afforf it for the Freeze infliction (to nuke with Pyromancer) or Ryuusei's Staff series if you can't.
For fire I suggest you get Arcane Cutlass of Combustion if you can spare the UR GGB, and if not - the fire magestaff, in order to maximize the damage of your Pyromancer Bloodmage.
For darkness, I would advise Blackbones as s spell booster, 100-proc magical damage weapon and a lifestealer.
Accordion Shield gives +20 Control Potency - which is by far the best item in the game for Mesmerize infliction.
Pie shield, as mentioned by LA, has a Cha boosting toggle to enhance your pets and guests.
For fire, I would suggest Shield of Agony's Blood. That -10 bth lean can come in rather useful.
Water is the best shield element. It has two very good shields - Celtic Wheel series with an Int toggle and Kindred, which can use charges to heal your mp.
Energy is meh. Taladosian or Grounded Flora are your only real options.
For light and dark you should probably use Sol and Luna shields which provide damage reduction based on your Cha or 100-proc weapon wielding.
Fortunately, you can free fire, wind, water, energy and light slots here due to tomes or Pyromancer, allowing for guest calls and summons.
Destruction Burst is the strongest darkness spell in the game. Be warned, though, that it has a small hp cost to pay for its power - that can stack up really quickly against tank-type monsters. Make sure to use it with Blackbones.
Frigid Fury and Gravity Crush from the Spell Scroll quest are the best spells available for ice and earth.
Archmage Researh, mentioned by LA, is your only real source of Harm damage.
Nezujimbo is the best ice guest available due to Freeze infliction (for your Pyromancer nuke). It has a small gold cost, but that's pretty much negligible.
Call EnergyZard from Frogzard Hunter Part One. The only energy guest really available.
Call Mana Bat, if you can spare a Rare GGB, calls a water guest that can be toggled to replenish your mp.
Finally, Purple Rain, which has the rewind ability (for both you and the monster) that allows you to regain the sp you used up on Mesmerize once per battle. Cast Purple Rain, Mesmerizes, then Purple Rain again.
Fairy Godmother is a good choice, as it can be toggled to replenish your mp, hp or attack. With Mana Bat, they can recover most of the mp you spent on a spell.
Skele-Brain mentioned by LA is probably the best darkness pet available. Nightmaragon becomes extraneous with Essence Orb.
Nilak from the Guardian Shop has Freezing potency - good for both your Nezujimbo and Ruusei's staff.
Underwyrmling from LA's post can be toggled to harm the enemy's sp, but is locked behind a fairly difficult boss. Your knight's revival will have to do for the moment.
If you use Kindred's shield, then the Kindred pet is very much worth it for rapid Meditation Charge generation. Otherwise, the Cephalopod is your only real choice.
PikaZard can be toggle to paralyze the enemy.
Your Psychedelic Dynosaur is one of the best wind pets available.
Hammer Turret is an earth pet that atempts paralysis. Morningstar can be toggled to randomly inflict either DefLoss or EleVuln.
Windter Crown is a good choice, as its the most versatile damage boosting misc available.
Goggernaut Helm from the Arena of Enthusiasts would be your bth misc of choice. It boosts the accuracy of your enemy - but also of your weapon attacks, spells, pets and guests; and the bonus isn't /2 for spells as with other bth boosting miscs.
Corrupted Djinn Lamp summons an mp-costing fire/darkness compression guest.
Gemini Pendant from the Account Shop for an earth/wind compression guest.
Paladin's Oath for an mp-coting light guest with a trigger against undead.
Your Power Shard or any other Int boosting misc for spell damage.
Morningstar Helm for paralysis immunity.
And, of course, Essence Orb so that you never run out of sp.