Hey Gamers! Welcome to NHL THREES in NHL 19!
Video below will show how to unlock this achievement in combination with Mascot Winner!
Basically we are going to play a game of NHL Three's in the exhibition mode against the CPU and set the difficulty to rookie or semi-pro. Select your team to be a mascot team (east or west) and then play the game. The first goal you score will net you the Costume party achievement, and when you win the game you will unlock Mascot Winner.
An easy peasy double wammy of achievements in less than 5 minutes! Hope this helps,
P.S. Props to Fifthmusketeer for writing up his guide. I just like to include videos for those of you who prefer video. His method is well-written and to the point, so if you prefer his text by all means give him a positive vote!!