Have the Blink gift (the lower of the two 50-point upgrades). Also have at least one dozen rounds of ammunition by scoring at least 60 on a previous attempt. Blink will let you teleport any distance as often as desired. Do this to avoid homing enemies, collect health, and dodging exploding clusters. You do not have to spend extra points into armor or life since because you will rarely take damage and almost always replenish it quickly by teleporting directly to a life power-up.
Play on level 2, "Angry" under the Very difficulty setting. You do not have to kill the enemy to use the door, however the outline will not be visible. Rainbow particles will appear from the bottom of the door. Enter the door to access the Super Secret Basket-Ballpit.