While I agree that our defending is so bad this year that the hungover lads from the Dog and Partridge would probably score at least two past us on a Sunday morning kick about, I must point out that Garde has been without Amavi since Man City and Richards has missed the last two games. He's been unable to select a settled back four.
IMO the only major tactical blunder he's made so far, which he himself admitted, was not playing a defensive midfielder against Everton. I've not seen anything else that you would question from a tactical standpoint, it's simply the ability/confidence/execution of the players on the pitch that are letting us down at the moment.