Another Pixie Strike question:

Last night in the Lake of Ill Omen, I was Pixie-ing Sarnak Adherants and
Revealers to interrupt their casting. It worked wonderfully.

However, when our group moved to Frontier Mountains to tackle Mountain
Giants, I'd try to Pixie if we got more than one pull but it would never
work. I got some strange message to the effect of:

You cannot mesmerize this creature (not with this spell)

This was on a blue Mountain Giant Hillock - they Charm easily enough. Now,
I know there's not a level limit on the song but is there a size limit? ie.
does it work on large creatures?

On a brighter note, I dinged into 30 and got a Forest Loop, so I'm happy.