The Short Version: Head to Hazy Maze Cave, find the poisonous haze area, and break the block with the Mushroom. Next, go back to the two Monty Mole holes and run in a tight circle over the moles. It won’t be long before you start racking up several extra lives before the Mushroom wears off.
After playing Mario 64 DS for a while I started searching on the Web for ways to rapidly accumulate extra lives. On YouTube I found videos that demonstrated a glitch that could be used to quickly gain extra lives. Unfortunately, it seems that glitch was fixed in the edition that I own. I found other videos that showed a method, this time not dependent upon a glitch, that works in Hazy Maze Cave.
This method should work with all characters though it’s easiest with Wario. It’s not very hard to pull of with just Mario. By the time you can access this level you’ll already have Mario as a usable character.
From the castle entrance take one of the two wooden doors that lead to the basement.
Next, head to the area that includes the painting for Lethal Lava Land and the hidden Shifting Sand Land entrance. In that area look for the door with a star on it.
Once through the door jump into the metallic pool to enter Hazy Maze Cave.
From the starting point take the branch on the right.
Next, time your jump and use a long jump to reach the door.
In the next section head down and then take the first exit on the left.
Head through that door.
Locate the hole in the floor and drop down. Be prepared to head right, out of the poisonous purple haze after you land. I recommend that you take out the Shy Guys – they may defeat your character before you can get to the Mushroom. Wario is the easiest to use – the red question blocks will give him metal invincibility.
Take the path behind the Monty Moles and then head right in order to reach the bricks that contain the Mushroom block.
Break the bricks and use the Mushroom.
Finally, head back to the Monty Mole holes and run fast, tight circles between the moles to gain extra lives.
After the Mushroom has worn off I usually just let the Monty Moles knock Mario down until he’s sent back out of the level. It’s faster than existing the course and restarting back at the castle entrance.