@Chuck80 this post is excellent. I decided it needs it’s own thread.
Also, this was in reference to getting various elemental versions of The Machine, but the elemental discussion in this post is useful for any gun.
Continuing the discussion from The machine! get it!:
Half flesh? So there are separate hitbox weak to fire element and the other weak to corrosive?
Thanks 
Maybe a small list is in order. Bosses in the game with their weaknesses and how much of their total life it represents (percentages are very loose approximations)
Flameknuckle: Shock 33%, Corrosive 33%, None 33%
Iwajira: Fire 100% (Cryo works)
Deadlift: Shock 90%, Fire 10%
Redbelly: Fire 100% (Cryo works)
Bosun: Shock 50%, Corrosive 25% (turrets), Fire 25%
Felicity: Corrosive 100%
Zarpedon: Shock 70%, Corrosive 15%, fire 15%
RK-5: Corrosive 100%
Powersuit Felicity: Corrosive 100%
Sentinel 1st form: Shock 75%, Fire 25%
Sentinel 2nd form: Shock 33%, Fire 50%, None 17% (Cryo works)
Despair & Self-Loathing: Shock 50%, Corrosive 50%
Denial subroutine: Shock 50%, fire 50%
Sponx: Shock 50%, Corrosive 50%
Shadow-TP: Shock 50% Corrosive 50%
Eclipse: Shock 50%, Corrosive 50%
Eos: Shock 75%, Corrosive 25%
Of all bosses life bars put together:
35% are weak to Corrosive
40% are weak to shock
only 22% are weak to fire, and mostly concentrated in just 4 bosses, 3 of which can also be cryo’ed
and 3% to NOTHING.
In absolute numbers: (discounting the time where a weakness represents less than 20% of the boss)
12 out of 17 are weak to shock at some point
10 out of 17 are weak to corrosive
6 out of 17 are weak to fire
Why Cryo is not so great against Bosses:
Many bosses are simply immune to status effects. And among those that aren’t, those that have a shield will be very hard to freeze while their shield is up. So Cryo is almost never a good choice against bosses. The only bosses where using a Cryo gun makes sense are RedBelly (after they split), Iwajira and The Sentinel’s large phase (once shields are depleted).
Why fire is not so great against bosses:
There are only 4 bosses where carrying a fire weapon would make sense: Iwajira, RedBelly, Bosun and Denial subroutine. In the case of Bosun and Denial Sub., they also have shields, so carrying a shock weapon is still necessary. Conveniently, the list is almost the same as the one above. So Cryo or fire as a 3rd boss weapon is more or less your preference.
I hope I didn’t make any mistakes writing this… If I did please correct me.
I mean that it has a health bar and a shield, so half of the fight, you’re shooting at flesh.
Am I getting a sense of how OP cryo is? 
I see. I thought they had some parts weak to fire and another to corrosive as Bosun has robotic (?) flying chair (?) and Denial Subroutine is a digital (?) thresher. Like how Armored Maniac back in BL2 has its head weak to fire and the rest to corrosive.