Trying to come up with a suitable AP fighter/duelist Champion

EDIT: Concept topic is complete - http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/aGuGtOEQ-celes-the-hand-of-justice

Please direct future feedback to the topic above.

I've been trying to come up with an AP fighter duelist Champion who's gimmick revolves around marking a single enemy and then dueling it to the death. She doesn't have burst like an assassin, but has good engage and what I call "trade superiority", in that she has an innate advantage in auto-attack fights.

These are the skills I presently have. NOTE THAT THEY ARE SUBJECT TO CHANGE.

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Celes, the Hand of Justice
http://danbooru.donmai.us/data/65452cde5c4f7af0c07e1034ea26bada.png
(I didn't draw this; only meant to be used for reference)

P – Code of Honor
Damaging enemy Champions with basic attacks or spells grants Celes a shield that absorbs up to 10 + (5 X level) (+0.1 AP) and lasts up to 1 second. Does not stack (normally); refreshes duration if reapplied. Attacking targets affected by Verdict enables the shield to stack up to 3 times.

Q – Zealous Charge
Celes dashes in target direction (600 range) and deals magic damage to enemies she passes through (half damage to minions). Celes stops upon striking a Champion. If she strikes a Champion marked by Verdict with Zealous Charge, she inflicts a powerful slow on the target.

W – Fanaticism
Passive: Celes’ basic attacks deal 15/20/25/30/35 (+0.1 AP) bonus magic damage.
Active: For the next 6 seconds, Celes gains increased attack speed and triples her bonus magic damage when attacking a target affected by Verdict.

E – Divine Punishment
Celes delivers a shield-bash to target enemy (short range, direct target nuke), dealing magic damage. If her target has its back turned to Celes, she snares it for 1.25 seconds and deals bonus damage based on the target's missing health. Hitting a target affected by Verdict changes the snare to a stun.

R – Verdict
Celes casts her judgment on target enemy Champion, marking the target and revealing it for 6/8/10 seconds (not including stealth). When Celes’ other abilities strike a target marked by Verdict, they gain bonus effects. Celes gains drastically increased movement speed when moving toward a target affected by Verdict.
If Celes kills a target marked by Verdict, the cooldown of Verdict is refreshed. Assists reduce the current cooldown by 70%.
800 casting range.

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Celes is basically meant to build stuff like Nashor's Tooth, AP tanky items, but not Infinity Edge or Bloodthirster. Think of her like Diana with less burst, better DPS, and better auto-attack capability thanks to getting a weak shield every time she attacks a Champion.

Anybody want to lend some helpful opinions? AP fighters seem to be one of the harder things to make.