I beat the game and did (or so I thought) everything I had to do, but I still can't get the 200 man melee. I have all the letters, beat the 100 man melee, etc. I must be missing something, but I can't figure out what it is.
Make sure you unlock it by achieving the prerequisites in the proper order:
1. Beat the game
2. Clear the EX Dungeon (get Krato's letter of challenge)
3. Obtain the other Letter of Challenges (Leviathan Claw, Flynn, Hunting Blades)
4. Beat 100 melee with at least three Characters
You have to beat the 100 melee with at least 3 characters AFTER getting at least 3 of the other letters otherwise that 100-man run doesn't count. Anyway, just do it in that order and everything should be fine. Good luck.