This is for "vs 2nd player" only, done by yourself and 2 controllers. Though could be done with CPU, I imagine this method is faster with 2 player.
EASIEST way to do this is...
(From main menu)
-BATTLE
-Vs. Player 2
-(pick any arena, I pick Avengers Tower)
-press
- Set the battle to these settings;
--Time: 30
--Damage: Highest
--Adjust your handicap to highest with
--Adjust player 2 handicap to lowest with
(with player 2 controller)
-Hit
to go back to fighter select
-Select any 2 fighters and stone for yourself and player 2. (I picked RYU because he punches fast)
Once the FIGHT starts you can either wait out the 30seconds for 50 matches, OR you can punch your way through 50 matches. Using Ryu and just quick punching I consistently finished each match in 7 seconds, but then had to wait maybe 10-12seconds for the cutscenes and rematch load. At the beginning you can press
to skip the little dialogue scenes between fighters which saves you a little time as well.
I mostly recommend this method if you have already played the game a bit and haven't popped this yet.
[When I did this I only had to do maybe 16 games and it popped, I'm unsure what other missions or battles I did that counted towards this. But don't be surprised if it happens sooner than 50.]
Hope this was helpful!