Save vs entangled is rolled by using gnolls stat modifier (this case strength). Unless they have listed stat to have proficiency in (and then you add proficiency bonus).<br/><br/>In regards movement all movement inside enemy threatened area causes AoO but casting action in itself does not cause AoO. Also DC 12 is good as save modifiers in low levels are between +0 to +5 (depending on stat and is there proficiency on given stat save or not). I doubt gnolls have much charisma or proficiency bonus to charisma rolls and depending on initiative gnolls may suffer additional d4 penalty to attacks and saves (at least few of them).