I've said this before, and I fully accept not everyone agrees, but for me, starting Giroud upfront is counter-productive if we want to play quick, incisive & effective football. Not only does he have poor hold-up play, he is too static. Defenders can push higher up the pitch and squeeze us because they know he hasn't got the speed/spatial awareness to get in behind. This will, in turn, put more pressure on our midfield and defence as they get pushed further back.
A central midfield duo of Xhaka and Ramsey was never going to dominate Wanyama and Dier. It would have been better to put Ox in the middle alongside Coquelin or Xhaka, and then put Bellerin as a wing-back. Our wide options weren't dynamic enough.
Starting Gabriel instead of Holding was just stupid, frankly. Especially as the latter, despite only making 10 appearances, has looked far more composed in every game he's featured in compared to the erratic Brazilian.
I'm not sure what Wenger's beef with Walcott is, but he has 18 goals for us this season, so he clearly provides a potent goal threat. To bring him on so late when the game was pretty much done and dusted was just pointless. Likewise with Welbeck who should have started or also came on a lot earlier. He's not the greatest finisher, but his speed, athleticism and movement can cause problems for defences.
Overall, it was just a catastrophe all around. A cowardly display from all involved. Not helped by the team selection as you say.