Taken from the in-game text:
"Every 3rd shot to the Left Tower Ramp takes the ball to the sorcerer's pedestal. The sorcerer gets quite angry when disturbed in the middle of his arcane research. When the ball hits the sorcerer's pedestal the 3rd time, a 3-ball multiball starts, and the sorcerer summons some of his creatures to block the kids' path to him. Defeat these monsters to get a passage to the sorcerer again. Maintain your 3 balls and shoot the Left Tower Ramp with each one to score sorcerer hits. The 3rd hit concludes the mode and grants an obsidian stone."
To see this text, press start on the table and select "Table Guide"
Great advice from Capta1n0bvious3:
It should also be noted that you have a 30 second ball saver at the beginning. Let all of the balls drain except 1 and try to quickly hit a few of these shots before the ball saver ends. Makes this a bit easier.
You may actually be able to hit all shots before the timer ends.
You can tell it's still going because of the blinking light between the flippers. If this goes out or stays solid ( cause you've earned an extra ball ) then the ball saver has ended.