Having tried for this several times, I have found out a few things:
1) It's best not to try this on the first go, but play as Knives first to unlock ALL levels while leveling her up to max (16) and all of her stats first so you'll have an easier time.
2) It is not possible to get this achievement in the same playthrough with
because of the method of playing first with your character on Average Joe to unlock all stages and then just replaying the parts of the game with the bosses to beat Supreme Master. I tried, and it didn't work so I went back and replayed it on Average Joe from start to finish.
3) You MUST do this in one sitting; you can't just paly halfway through and then stop and quit the game and come back later.
Now, here's the set-up:
on the title screen to unlock the Power of Love Sword.
Second, as stated above, play as Knives on whatever difficulty you wish and level her out to Max, including her stats. Here's a guide I found on how to up stats without having to grind for Wallace's Mystery Shop or the Video Store method:
Sushi Bar (World 1, Shopping District)
-Buy 4 Avocado Hosomaki ($2.50 x 4 = $10.00) to boost your SPD by 4
Shock Wave Record (World 1, Shopping District)
-Buy 12 Smashing Turnips albums ($16.95 x 12 = $203.40) to boost your STR by 96
-Buy 12 Crash and teh Boys albums ($14.95 x 12 = $179.40) to boost your SPD by 96 (making it 100)
Scott's Dad BBQ (World 5, Fire Escape)
-Buy 2 Kebabs ($4.95 x 2 = $9.90) to boost your STR by 4 (making it 100)
Mobile's Secret House (World 6)
NOTE: This is hidden in the beginning of the first level; you need to go through two trees that are between 4 people loitering around. Be warned, sometimes this won't work right away for some reason, so keep trying since you'll need to go tehre anyway for both the Special Training item to buy, and for the food items you need for another achievement.
-Buy 25 Sauteed Shiitake ($9.95 x 25 = $248.75) to boost your DEF and WP to 100 each.
Finally, once you've beaten the game the first time around and have all your stats maxed out, start from the very beginning and go to town on the enemies. With the Love Sword (save for the boss fight with Todd and Techno Base part), and your stats maxed out, this should be a piece of cake.
Use the Subspace Highways to help move along quickly, and mostly ignore grabbing dropped money. I say mostly because you might need to buy some extra food at the shops along the way, especially towards the end.
One last tip: If you have extra dough when you start the final full playthrough, grab some 1-ups for emergencies.