It’s kinda broken to be honest. 
Basically : Bloodsplosion’s Damage is based on Overkill Damage, and it is boosted by Grenade Damage as well. By triggering Fuel the Blood just before the fight, I get a massive +220% Grenade Damage boost that essentially comes twice in the damage equation (once in the Overkill Damage itself with the Fastball, and another time in the actual Bloodsplosion damage).
I hope you don’t mind some math : thanks to this awesome website I could find how much health does a Sewer Spiderant have. They have 1.81 million health.
With Fuel the Blood active, the Fastball does : (base damage * Fuel the Blood) * Bone of the Ancients * Elemental Multiplier * Slag
Replacing the values, we get : (27081114 * 3.2) * 1.39 * 0.9 * 3
= 260186677
And multiplying by the 20% Damage Reduction Level 75 enemies get, you have an actual damage of 208149341 (which is close to what you can find in the video, it’s not exactly so because I didn’t have 100 stacks of Bloodlust) :

Knowing the Damage we deal, we can determine the Overkill Damage and multiply it by the various bonuses it takes advantage of :
A Sewer Spiderant has 1.81 million health, so we can directly substract it from the damage we dealt to it : 208149341-1810000
= 206339341.
Then, we can calculate the first Bloodsplosion chain’s damage on Pete :
(206339341 * Bloodsplosion multiplier) * Fuel the Blood * Bone of the Ancients * Slag * Elemental Multiplier
Replacing the values, we get : 206339341 * 6 * 3.2 * 1.39 * 3 * 2.5
= 41300882494
And with the Level 82 Damage Reduction Pyro Pete gets, you get : 41300882494 * 0.45
= 18585397122 > 999999K !
And then the second chain kicks in …
(18585397122 * 6) * 3.2 * 1.39 * 3 * 2.5 = 3720053087939 ; 3720053087939 * 0.45
= 1674023889572 !!!
So that’s it. That’s why you can waste some of those Raid Bosses so easily using Bloodsplosion. I had to calculate all this beforehand to know if I could pull off the kill with only one valve opened, but I deleted the file like an idiot so here I am doing it all over again ! 
Edit : just realised I didn’t multiply the Bloodsplosion values with the Shock elemental multiplier, so the damage is actually even higher !