The Cannon Boss (Troops 903) is bugged (Incubus War, LoB Island). from the gamefiles it says the boss is supposed to use Cannon Blast at the end of turn 2 (battle event 3). However, if you kill the Blood Underlord or either of the Blood Brutes it only deals damage to your party and doesn't self-destruct (event 3 from Troops 903 doesn't happen) and instead only uses the Skill Cannon Blast. Also, killing any of the 4 Blood Veterans doesn't prevent the self-destruction, but makes it so that some of them survive the self-destruction.
I found a solution to this: Change the code of Troops 903, battle event 3 line 2 from "Change Enemy HP: Entire Troop, -99999" to "Change Enemy State: Entire Troop, + [Defeat]"
I've tested this about 20+ times and they all properly self-destructed (as they should)