Hmm. I find it endlessly frustrating that there are all these exceptions to the rules that the player is never made aware of. Thanks for solving that mystery though.
Tremor is a ground target AOE. It's the only troll attack that sparks cannot make to miss. But it is still easy to handle - the radius is shorter than your melee range, so you can stand just outside of it and continue to melee the troll who basically stunned himself for several seconds.