Note that using Ajani's abilities on the Phalanx Leader will NOT trigger heroic. As such, you're only sitting on four triggers for its ability. Without adding some added sources to trigger heroic, you might as well just drop them for 4x Dryad Militant and swap the Ethereal Armors for something else (Imposing Sovereign?) going back to the mono-white aggro that people have been playing for the past few weeks.
Now if you want to keep him in there, you could easily slide in things like Gods Willing which can serve as spot protection (so you don't go wasting Brave the Elements to save a single creature), a source for some scry, and a heroic trigger. I've actually put together a build similar that uses Swift Justice, admittedly not the most commonly played card, to offer another 3 triggers while also keeping him alive in combat and gaining a little additional life. I can honestly say that I've tackled some tier 1 competition in the past using totally off the wall stuff like this before - it's a LOT of fun to drop Geist of Saint Traft with a Blood Artist care of Vampiric Fury... you can believe he didn't see that coming.
You can see the build I've been tinkering with here: http://deckbox.org/sets/489034 - keep in mind that my decisions were motivated by a lack of having but one Fiendslayer Paladin which is hands down better than Fabled Hero or just about any other three drop for white.
*You beat me to the response... so disregard the part about Ajani not triggering...