Given odds by book markers don't guarantee that Arsenal will surely win this match even they are the host. Real Madrid in UEFA Champions League is something really different and many times unexplainable. They can play not impressively and even bad, but still manage to win their opponents in knock-out matches.
Arsenal don't have mentality of a champion that is their biggest weakness when facing with a strong and very experienced opponent like Real Madrid. Playing with their best, full focus during the match will increase chance of having a draw or win for Arsenal. If Arsenal players only lack of focus in several minutes, Real Madrid will not waste those precious minutes to beat Arsenal in England.
Arsenal did this is ongoing season of EPL, after doing all the hard work they started giving away points when they needed them the most. Result is that they now have to settle for second position in this EPL season also. If Arsenal is team that can perform at crucial stages of the tournament then this is the time for them to prove. Though odds are in favour of Arsenal but still they must play the game with full dedication because Real Madrid is a tough competitor. Just few hours left in the game and I expect an interesting contest between two great teams.