Instead of playing through almost the whole game to get your character to level 20 you can use a sweet shortcut.
You do need the "Witch Hunt" dlc to do it this way.
If you have it just start a new game in the main menu. Select Witch Hunt and create a new character. Pick the character class for which you need the Trophy and start. Immediately at the beginning the Trophy for reaching level 20 should pop since the dlc levels you to 20 straight away.
Of course this also works for: