1) Barcelona Win. This is obvious, but a victory at Allianz Arena is the safest way to book a place to Berlin. Any result works, doesn't matter what the goal differential is.
2) Draw. The first tiebreaker is points total: with a draw, goalless or not, Barça will finish with four points in the tie, which will also give them a ticket to the Olympiastadion.
3) Two-goal loss. This is where things get good for the Blaugrana. Because of the giant 3-0 win they got at Camp Nou last week, Barça goes into Allianz Arena and will advance even if they lose by two goals. Any result works: even a 10-8 scoreline is enough to put Barça in the Final.
4) Three-goal loss with away goals. This is the scenario every Barça fan desperately wants to avoid, because that would mean losing all of their nails. If Bayern is able to reach a miraculous 4-0 lead that would qualify them to the final, Barça would need at least a goal to put the Away Goal Rule into effect: starting from 4-1, any three-goal defeat in which the Blaugrana score will be enough. But please, Football Gods, don't make that happen.
The only way this game would go to overtime is if the hosts are able to keep a clean sheet against 'MSN' and Co. and win 3-0 during the first 90 minutes. That would send the game into an extra two fifteen-minute periods. If nobody is able to score during the extra 30, then the spot on the Final will be decided via penalty-kicks, also known as "REALLY? DO I HAVE TO HAVE A HEART ATTACK NOW?!".
These are the scenarios. Let's hope things don't get too tricky and Barça is able to book a flight to the German capital without much troube.