Swift action: A Human Spirit Passing By.
Standard action: Spirit Sign "Abundant Floating Spirits". Crit threat increased by 4+3x3=13.
Attack roll (threatens on a 7-20)
Rolled 1d20+15 : 18 + 15, total 33
Damage roll
Rolled 2d4+4 : 4, 4 + 4, total 12
Each hit devil has three spirits spawn adjacent to them, plus a line-shaped spirit from A Human Spirit passing by.
Then move action to activate Slave Spirits and make them move 0 feet and detonate (auto-pass the necessary skill check). Each explosion allows a Reflex save with DC 21 to negate.
First Devil:
Rolled 1d20+23 : 16 + 23, total 39
Rolled 1d20+23 : 17 + 23, total 40
Rolled 1d20+23 : 2 + 23, total 25
Second Devil:
Rolled 1d20+23 : 7 + 23, total 30
Rolled 1d20+23 : 6 + 23, total 29
Rolled 1d20+23 : 7 + 23, total 30
Line spirit for both devils:
Rolled 1d20+23 : 16 + 23, total 39