Good news! The description is incorrect on the achievement
As stated in the "Unlocks" portion of the game menu, you only need to reach level 50 with Antonio to get this
First, you must have unlocked the Randomazzo, which is located in the north part of Gallo Tower. It looks like a deck of cards
Getting to level 50 with Antonio is relatively easy if you have advanced this far into the game and have spent some gold on Power-Ups. He has the whip as his beginning weapon.
Head to the Mad Forest if you want to keep things simple. Any combination of weapons that has been successful for you up until now should keep you alive until level 50. There are many fire braziers on the map that can drop turkeys to give you health back if you are in a pinch
I have found that garlic is very strong against the enemies you will face in the first 15 minutes of the map. It gives you enough protection to allow you to move to the gem piles your other weapons are creating. If you want to get to 50 even faster, consider the crown for XP boost when it is an option