Ravanna in Myth III does not seem to be a battlemage, if the myth world has such things, since the avatara fight very nicely.
On a general power level:
Mazzarin
Alric (By Myth 2)
Conaught/Balor
Alric (In Myth 1)
The Deciever
The Watcher
Soulblighter
Shiver|The Nine
Correct me if I made any blindingly stupid mistakes.
So Mazzarin would be at the top I think.
But when the Watcher killed him,
he must have had thousands of Thrall which wore down the great avatara, and then the sight of him and the final wave just as he (Mazzarin) was weak was just too much for him.
So he lost.