Ice Cave of Sorrow Mission

I've tried this mission for about 10 times... but everytime Evenna stays behind and tries to kill the first foe she sees...then got nuked by a hord of Muscaats.

Can anyone give me some tips on this mission?

Oh...found someone who did it with me. Seems u wait for the Mursaat to pass...then they will get themselves killed by the seige weapons.