With Arsenal's players, you could honestly play anything. Considering the beta has dubious defensive AI, I'd recommend three at the back and whatever you fancy ahead of them as right now it's all about scoring more than the opposition. Mark and close down the opposition Full-backs, as a lot of the opposition moves start with them. Do whatever you can to maximise your team's possession, but with a 3-5-2, 3-4-3, or 3-6-1, I'd play very fast. Have your D-Line play narrower, as currently the defenders in any formation tend to spread themselves too wide. I play an assymetric 3-2-2-1-2 with serious closing down, and I'm annihilating teams now that my tactic familiarity is high. You may concede a goal or two in a game, but if you've bagged six yourself, it hardly matters.