There is no way to level up fast and become really strong. Unlike Divine Divinity, this game is more linear and you gain xp along the way. However, it is possible to miss experience by not finding all the quests.
Don't level off monsters that are hard to kill, chances are if you are attacking real hard to kill monsters (where two kill you off) then you most likely missed some lower level stuff. I've found dungeons full of goblins so late in the game that they didn't give any xp as I was too high of a level.
As for the saving thing, I think it be too much work to be worth doing. Better off to save before getting a quest reward and keep loading the game until your offered something good. Same goes for shops when they restock.
