shim: On the optimum path, there's another ally on the field. Have him attack Kudlak - he cannot kill the vampire, but he can drop him to 1HP. Direct Mari to him then. If you're not following that path, surround him with everyone you can, and whittle him down. That battle only really determines whether Mari will be playable on Day 7.
As for 18:00, just do the Honda event. If you are facing Belial, he'll show up at 18:30.
On topic, after blazing through 358/2 Days, I'm blazing through Spirit Tracks. I'm after the Compass of Light now, so I assume I'm at endgame already.