Brill was using the Fire Formula III version of the Boom Trap (which has 120 base damage), and it looks like Brill was sitting at 25 ATK, with 76 damage taken meaning +7 Attack from a Ring of Reprisal, a +3 Rock Wall and 7 Stone's Throws adding another 5, and 170% Fire Amp (Fire Affinity and Sage Cloak) on top of a 7% boost from a Fairleaf Laurel. (I went here straight from an Aeternator attempt, Brill was kitted out to deal damage and deal it fast.)
That's 157 damage, minus 10 from the Corsairrot's defense = 147. Multiply by 1.7 and then by 1.07 and that's 267.393.