For this achievement you must kiss Nick, (Juliet's decapitated head for a boyfriend) 100 times. This is a pretty time consuming achievement as it will require the usage of 100 Nick Tickets! This requires you to use a Nick Ticket in order to pull off which can be found throughout stages or purchased at the Chop2Shop for 30 medals. Only 9 tickets can be held at once. Each time you use a Nick Ticket by clicking a roulette game will come up. When you successfully land on a Nick Move a cutscene will show and Juliet will kiss Nick once. Some Nick Moves must be purchased before you can use them. Use enough moves to kiss him 100 times and this achievement will unlock. I recommend you try to use tickets once per stage section and use Nick Shake to help work toward Millionaire Hunter. Below is a nice grinding method for medals and using Nick tickets.
Save up some zombie medals and start up Stage 5 (Cathedral) on Easy. Get through the first part of the stage where you blow up the truck. Use the combo , , to kill the first zombies and go into Star Soul Mode for extra medals. There's a shopping kiosk very early into this stage so that you can grind for this. Now use Nick Shake to get around 120 medals then go to the auto-save point (right passed the Chop2Shop), you should see the save icon. Exit out to the main menu then re-start the game to continue from last checkpoint and have the Nick Shake move back. Now get to shop again and repeat the process of buying and using Nick Tickets.