Scott Pilgrim vs. The World: The Game

Replay the core game again as Wallace. This can be done online or offline, and you can even do this on the Average Joe difficulty. You do actually need to play the entire game, and skipping to the final boss won't work.

Note that while all stages are available to select online, the online progress does not save into your offline story mode. For example, if you completed the first three stages offline, and play stage four online, when returning to your story it will only show you have completed the first three stages (and you can only continue from there or earlier). You can however complete the remaining stages online. Using the above example of the first three stages completed in offline story mode, you can play stages four to the end online and get the trophy!

If you want to make this real easy on yourself, you can use the money and in-game cheats to do this even quicker. These cheats can only be done offline, however your overpowered character can then be taken online:

Money/overpowered cheat:
Complete stage 1 on story mode (offline) with Wallace if you haven't already. Now start up a local 2P game with Wallace on controller 1 and on controller 2, pick guest profile with whatever character of yours has the most money. Select the Shopping District (Stage 1.5) and jump into the video store. Have controller two hold down to lend all his money to Wallace. Then exit the video store, return to map (this saves the money for Wallace but leaves the other with the original amount) and quit the game. You can repeat this as often as needed, but you need less than $1000 total to get everything you need. Now have Wallace jump into the video store again, pay Scott's late fees of $504.50 to be able to rent movies at $4.95 each (in game money). Purchase as many Alone and Disturbed videos as you need to hit level 16. Then get ten copies of Seven Shaolin Masters to raise your stats to the max (each purchase adds +10, and max is 100). Lastly, grab up to nine copies of The Mystical Head as each one will add another life (you can have up to 9).

Boss Rush mode (only works for offline games):
When at the main title screen, press:

Heart Sword (only works for offline games):
When at the main title screen, press:

(There are other cheats as well but these are the ones that make it easier/faster).