There are many methods to earning extra lives. You can collect 1-Up Mushrooms, collect 100 Coins, or stomp eight enemies in a row. However, there’s no way that’s faster than, or accessible as early as, this method, which you come across in World 1-2.
Not long after entering the cave, you can drop into this side area, where a Koopa Troopa patrols alone. Take its Shell, then walk to the mouth of the nearby passage. Position yourself with the Shell as Peach appears in this screen shot. You should be Super size for this.
Kick the Shell into the wall while jumping simultaneously. You’ll probably get hit in the process (which is why you want to at least be Super sized), but you’ll settle into a repeating pattern of bouncing off the Shell and bouncing into the roof. You can make this a little safer by diving into the shell, start bouncing between walls, then jumping out of the Shell. You don’t even need to hold your controller at this point! You can accumulate the maximum of 1,110 lives within a few minutes using this method.
This is incredibly useful for playing with friends. Multiplayer Super Mario 3D World can tear through a stock of extra lives very quickly, especially on difficult levels. Having so many extra lives can give players of all skills plenty of leeway to get through difficult stages, and can make up for any unfortunate “accidents” that may occur during multiplayer.
Another notable location to pull off this trick is this hill on the beach in World 5-1. The passage you must enter for the Key Coin can be used in similar fashion, and there are plenty of Koopa Troopas on hand who can donate Shells to the cause.