while it is nice to have a 300 AP to maximize your power i think lvl 5 mage does not belong in the 300 AP bracket. My fear is that i can simply rush your mage as you just have one (poweful) knight.
comps that will wreck you (imo):
-anything with ferocious...even 2 archers (72 AP value) can potentially wreck your knight. Morbid puppets can also wreck your mage by themselves. In my Ram band i usually end up with 8 units (2 are morbid puppets) and i see it for you very hard to control them all with only the knight. Yes probably the mage will kill 1 or 2 by itself but it will go down.
-wolven can do the same more or less and you might even be at numerical disadvantage. One fang warrior can quickly kill your mage in 1 turn with mightly blow and bloodlust.
-other lions with favorable auspices (valk+archer, which is pretty much in any lion comp right now) can take your sparkling down fast, followed by the mage. All from a safe distance.
my idea is:
better to use a lvl 3 mage with recovery and seal of dellar'n if you plan to use lot of sparklings (combo'ed with the seal they are so nasty it is basically your favorable auspices) or other cheap units at 300 AP.
No point to use such a high level knight also, again level 3 might be the best in terms of AP.