As he speaks to them he pulls The Hat off his head and holds it by the tip. Smoke begins to come out of the bottom as he rears back and whips it over his head like a sling. A ball of light appears and flies into the air, growing larger as it does so. A baseball, a grapefruit, a bowling ball, a beachball, bigger and bigger until it falls into the fortress once more. A massive explosion erupts, blasting all of them with fiery intensity but leaving them unharmed (minus a bit of charring and a smell of smoke on their clothes). The orcs, mages, and trolls, however...
The fire elemental seems completely unharmed by it. The living fire spills over the edge of the fortress onto the ground and quickly roared, spreading across the ground before coalescing inside the fortress once more. It leaps onto one of the orcs there, quickly catching onto what's left of their smoldering form.
Presto casts a level 7 fireball, centered onto square F11. That hits everything except the orcs on B7 and I7. All of those creatures need to make a Dex save, DC 21 (half damage if pass). This uses another 7 charges of The Hat (36 remaining).
EDIT: Sad fireball damage rolls. The second "Touch Damage 1" should've said "Touch Damage 2".
Fire Elemental: 125/132 HP
Rolls
Level 7 Fireball (Dex Save DC21 for half) - (12d6)
(451121524522) = 34
Fire Elemental Touch damage (fire damage per round) - (1d10)
(6) = 6
Fire Elemental Touch Attack 1 - (1d20+6)
(12) + 6 = 18
Fire Elemental Touch Damage 1 - (2d6+3)
(25) + 3 = 10
Fire Elemental Touch Attack 2 - (1d20+6)
(20) + 6 = 26
Fire Elemental Touch Damage 1 - (2d6+3)
(16) + 3 = 10
Fire Elemental Touch Damage 2 (crit damage) - (2d6)
(63) = 9
Fire Elemental fall damage - (2d6)
(43) = 7