When starting a new world, go to the "Reports/Competitors" tab, and find out which competitor has the lowest cash/net worth. Then, go to the "Marketing" tab, and hire some Goons. After a few days, you may have to visit the loan shark, but it is well worth it. If you hire four Persuasion Trolls (or Three Persuasion trolls and a Giant Lazy Baby, which seems to work the best) and set them directly in front of the weakest competitor's store, the competitor will quickly run out of money and will be driven out of business. Once they are gone, repeat the same tactic with the next lowest competitor until they are all gone.
Fairy Godmother Tycoon