Item description:

Shopaholic
Buy every upgrade and character in multiplayer.

Just buy the characters from the enemy faction. (if it doesn't unlock the trophy immediately, exit the game and restart) I unlocked it by restarting the game while loading the trophy and I was given.

These are the longest trophies as far as the online sector is concerned, the rest of the trophies can also be done in a game with 2 or 3 friends.