i have come from icewind dale 1, where you are able to mine xps in order to get stronger for the tough bosses.
in nwn2 it seems that missions are tight and do not permit those tactics.
any ideas?

Not sure if you'd want to. I followed the main path, ignoring some sidequests, had little trouble with the boses and ended the campaign with a lv20 character.

That aside, except for resting in an area that triggers sleep interuptions (some places in the end do this particularly often) i cant think of any.