Receive all the boss trophies and subspace emissary enemy trophies also beat all 10 bosses in boss battle mode on all modes.
Get gold trophies in all cups.
Unlock all the other trophies, then you will get the platinum.
unlock all other trophies in a game
To get the platinum trophy you must unlock all the other trophies.
Play more brawls in group mode and unlock all the characters and collect more trophies and stickers.
All character trophies are obtained by completing classic mode and all star mode.
by completing the game and finishing all the puzzles and mini games
You just get it out of random. If you REALLY want it, then I suggest for you to keep on doing All-Star or Boss-Battles (collect trophies that appear in the rest area), or Coin Launcher. I'd recommend the first two options, though, if you don't want to waste your coins. If you haven't unlocked those modes yet, then unlock all the characters to unlock All-Star and beat both the Subspace Emissary to get Boss-Battles. Good Luck!
you have to beat 150cc with all gold trophies to unlock 150cc retro mirror mode and then beat 150cc retro mirror mode with all gold trophies
You get trophies by doing time trials in Dirt Valley. If you get a trophy, you unlock the next track.
get 11 trophies.