Arsenal trail the Champions League tie 3-1 from the first leg and must score at least three times in Bavaria to deny their hosts a place in the quarter-finals.
No side has managed that all season against Bayern, who have conceded twice on just two occasions and go into the game on the back of 11 straight wins.
Arsenal travel without Jack Wilshere and Bacary Sagna, but Wenger told the club website: "It is not mission impossible.
"It can be done, but first of all you need 100 per cent commitment, attitude and things to go your way. But usually football goes your way when you have the right attitude.
"There is a kind of feeling of 'let's really play with freedom'. Let's start strong. We have to create doubt in their mind and you only do that if you have a real go.
"The chance you have is to make insecure the certainty Bayern have at the moment. That you can only do if you perform at your best.
"We can have a real go without being silly. We can't think that the game lasts 30 minutes and throw everything forward from the first minute on. We want to be positive but also intelligent."
Arsenal have not played since defeat at Tottenham nine days ago left them seven points behind their neighbours in the pursuit of Champions League places for next season.
Wenger said: "We have practised a lot and prepared for this game. It came out of a very stressful period.
"Mentally it was good to have a little breather and refresh. And on the other side, on the football front, we could mix up recovery and preparation."