It's hard to imagine a game between these 2 sides going UNDER. It's also easy to go Over here since both Tuesday's CL games went Over. The obvious doesn't always happen. Barca may be a great team but their offense is easy to solve on the road. Arsenal's defense may not be that great but a little "step up" will do here.
Two possible scorelines:
Arsenal 1-0 Barcelona
Arsenal 1-1 Barcelona