Same problem here, just one rush crit and 4 normal hits and she died.

My normal attacks were about 2300-3500, and 10k crits with battle rage.
Besides having full life leech, one hit means about 500 health?
Pop up a ghost and a creature with rush and the fight is even easier.

I was rather disapointed with the strenght of Belihir, i mena comon he's one of the great mages and he felt significantly weaker then Belegar!(unless i only killed a spiritual clone)