I was wondering if Shanna can be beaten when you 1st encounter her if over leveled.  I remembered on my 1st play through she wasn't able to 1 shot my level 28 Mirel. I am curious if beating her on the 1st attempt(without trapping her in a hole) was possible.

It's not intended that you defeat her, and doesn't give you anything special. The lose to her is a scripted event.