Instructions
Play the game until you talk to Tom Riddle. From there, kill the snakes that appear in front of you with the Reducto Curse.
Jump onto the platform that appears. In front of you is four tunnels. Behind one of these tunnels is the Basilisk.
Use the Wingardium Leviosa spell on the tunnel with a slightly silver tint around it. This is the tunnel containing the Basilisk.
Walk through the tower, and the battle with Basilisk begins. Attack him with the Reducto Curse. At times, the Basilisk disappears and a few snakes appear. Kill them to bring the Basilisk back.