I think lvl 13 is fine for act 1 from what I've heard. Kiya or someone who's actually completed the game will be of more help though.
Apparently you get better spells etc. in act 2 so I would soldier bravely on.
I'm far from an expert but i'm at the last enemy in the game at level 29. Nothing has been a problem at that level and i've seen german version players state they completed it at that exact same level.
I was level 12 in Act 2 and was fine, just be careful of the Earth Elementals (they earthquake which is very very powerful and would kill even a lvl 30 if a few do it at the same time).
Experience is very fast in Act 3 onward. I leveled without even thinking I was close, just completing a few quests.
