I've been using "Summon Arniel's Shade" a lot, usually right when I'm first aware of an enemy, often even to lure enemies out. But my conjuration skill doesn't seem to be leveling fast: it's at about 74-75. I think it's only leveling when I cast Soul Trap, Bound Weapon, or Summon Atronach spells.
Is this because XP is tied to base cost and Arniel's Shade costs 0 magica? Or is it because I have to start a fight first, then cast the spell?