I believe there is massive chance to hit penalties imposed on characters fighting a monster that is higher level than them.
For instance if you are level 3 trying to fight a level 6 monster you probably will hardly ever hit it.
Attacks of opportunity is a talent you have to grab 1st and allows you to perform one if a monster shows his flank to a character in range wile moving somewhere (I think)
Really wish some people would help out with the wiki
http://divinity.gamepedia.com/Divinity_Wiki