1) Just ask interact with it and win the RPS "game".
2) I found the last boss just way too easy and am happy that they ended up buffing them with the last patch. Almost everything that you kill gives you XP. You can finish their quests and kill them afterwards if you really want to get another level or two. During my playthrough I have reached lvl 22 without killing anybody in the towns, although I have attacked most of the opponents that would initiate a fight if you lose the Charm/Intimidate/Reason minigame after earning the Charisma XP.
3) You can cast haste spells during the battle that temporarily cancel the slow effect.
