If you're wanting to do this risk-free, the tightest spread I've found was on the Over 0.5 goals market. 1/25 (1.04) on B365 and lay it on Betfair at 1.06 for a guaranteed loss of £1.08.
Think I'm going 2/3-0 Bayern for the in-play. The last twice they've faced each other in the KOs, Bayern have won the first leg by a 2 goal margin and Arsenal have been pretty lacklustre up front away to the better sides this season.