Something for consideration for the Guardian "wish list" for a future update:

If it is feasible, are there any chances that Challenge might be made an immediate skill with a future update to the Guardian class?

There are times in raids where it is critical that this skill fires exactly when you want it to in order to "intercept" specific mobs as they head toward the main group (gathering up the Deep Haulers from opposite sides of the room and quickly separating them from the Taskmasters in the Lightning Wing of Orthanc is a prime example). As it stands right now, the time that it actually takes Challenge to "fire" is unpredictable, which can lead to occassions when all the Guardian can do is watch the desired mobs run by them into another group of adds that you don't want to get caught in the Challenge while you wait for the skill to go off. This variability makes it difficult to quickly and strategically separate groups of adds from one another, which can be a pretty significant part of various raid pulls.

If an immediate skill isn't feasible, perhaps making Challenge a fast skill would help so that the timing at least isn't as variable based upon the animation time of the skill that was used prior?

While the variability in the time that it takes Challenge to fire is certainly not game-breaking by any means, could this be a fix that might be worth looking into to allow Guardians to exercise more precise control over our ability to manipulate aggro?