I'll ignore, for now, the conceptual problem that I have with the Ardent (namely, the disjunct between its flavor as a "mood reader," or empath, and its mechanics as an exclusively weapon-wielding melee class). Fixing this would be an undertaking far too large for me to attempt, seeing as it would require a complete re-working of the powers to accomodate ranged implement psychic attacks, and wouldn't see publication anyways. So I'll grudgingly make my peace with the fact that there's very little in the mechanics that reflect the flavor well.
This leaves the AC problem. I'm assuming that the Ardent is going to be published as-is, so there's really no option for giving them shield proficiency out of the box. Pity, as that would have been a simple solution and provided an incentive for Ardents to wield something other than a polearm. This leaves a feat patch as a solution. There's precedent for feat patches (see Expertise feats), and there are also existing feats that serve as de-facto patches to compensate for design flaws (Chainmail Proficiency for Con Shamans). Sure, Scale proficiency or Light Shield proficiency could be taken to patch the Ardent's AC, but both require 13 Str and Scale requires 13 Con as well. So as it stands, improving the Ardent's AC exacerbates the inferiority of the class, as it requires a more MAD build. Thus, a new feat is in order.
Prerequisite: Ardent Mantle
If your off hand is free, you gain a +2 feat bonus to AC against all attacks originating from within your Ardent Mantle.
I'm assuming that the Ardent MC feat doesn't grant the Ardent Mantle class feature, so there shouldn't be any cheese with other classes stacking the bonus on top of their already respectable AC. Fluffwise, it fits the concept of an empath in that as an enemy is attacking, the Ardent projects a pulse of emotion into their mind to suppress their volatile temperament, reducing the efficacy of the attack. It also makes further use of the Mantle, which seems to be the Ardent's defining feature.
By going the Swordmage Warding route and requiring a free hand, deciding between a two handed weapon (likely a polearm) and a one-handed weapon is now a meaningful choice. The reason why I made the AC bump +2 instead of +1 was because A) +1 AC may not have been a good enough incentive for an Ardent to dump the polearm, and B) since Ardents should have gotten light shield prof. anyways, a +2 bonus represents the same investment that a Warlord would make to get heavy shield proficiency (1 feat, with the Str prerequisite being irrelevant for a Warlord).
Furthermore, there are tradeoffs that don't make it strictly superior to getting shield/armor proficiencies. In the case of Quell Savagery vs. lt. shield proficiency, the shield nets you a +1 to AC and Reflex, but requires a 13 Str. Quell Savagery gets you a higher AC bonus and doesn't require a Str investment, but it doesn't improve your Reflex and only applies to attacks originating within your Mantle (leaving you more open to ranged attacks). In comparison to Scale Armor proficiency, Scale would get rid of the armor check penalty that the Ardent keeps if he/she remains in Chain (with Quell Savagery). Furthermore, Scale may be attractive in that it offers enchantments not available to Chain (but once again, requires a Str and Con investment).
Overall, while I'm not a fan of "feat taxes" I'd rather have an option like Quell Savagery than requiring a class to become too MAD to fix an AC that's simply insufficient for a front line leader (and secondary defender, to boot). This way, a Mantle of Clarity Ardent can invest stat points in Cha and Wis and then choose to bump either Reflex (and possibly initiative) or Fortitude (and possibly surges) without being pressured into putting points in Str or wielding a polearm. This is especially important since new players might not see the necessity of these two courses of action, and will feel very squishy if they run around with a longsword and chainmail. At least Quell Savagery would be a fairly attractive feat choice for a new player, and it would be right in the first place that they often look for feats (the class feats list).