Instructions
Open your map. Click on Mourningwood to fast-travel there. Enter the fort, climb up the stairs and walk toward the mortar.
Talk to the ghost, Jammy, who stands next to the mortar. Say "Yes" when he asks if you want to play. You automatically enter the mortar.
Sweep the camera quickly back and forth across the area to see where the first group of Hollow Men is appearing. When you find them, blast them with the mortar.
Listen to Jammy's hints. Every few seconds, he tells you where a group of Hollow Men is appearing. Turn in the direction he gives you to find and kill the Hollow Men.
Fire the mortar only at Hollow Men in groups of three or more. The mortar is slow to fire and recharge, so it isn't worth your time to fire at a single Hollow Man. If you always fire at large groups, it is possible to score well over 2,000 points in 200 seconds.