I must say Bayern Munich defense far from what it was before to be honest, this is an obvious fact.
Anyway I think the beat way to defeat bigger teams is with a good defence and a good tactics whereby if the underdog can maximise their chances can get a win or at least a draw against any top team. Feyenoord pulled this off all eyes will be on them in their next match though which is against Lille, the two teams are a dangerous one as an underdog in this tournament.
Let the two underdogs meet so that we can see what the outcome of the game will be. They have been playing exceptionally well in the champions league and their performances have paved way for them to have come this far. Bayern Munich defeat was a big surprise but underdogs are very hard to predict and until when the damage is done before you’ll realize where the mistakes are coming from. Feyenoord may not be able to withstand Bayern Munich for the second time if they meet again, but for this meeting, they already outsmarted Bayern Munich and won their biggest points in the competition against a big team.