Gruff moves up next to the specter, and slices at it with his blade.
Booming Blade (1d20+9=29, 1d8+5=7, 1d6+4=7)
Crit for 13+1d6=3 = 16 damage, cut to 4 by weakened and insubstantial. All squares next to Gruff are difficult terrain for the specter, and it takes that extra 1d6+4 = 7 damage if...