Does Ultimate Inspiration work on a Giant Monster?
no because of how the scaling code works
it doesnt matter how many lvl shifts you have, you will always fight the monster as an even con

Right. In effect, your shifts are calculated first, and then the GM scaling is calculated. That means that the only effect it's possible for a level shift to have is to change where you are in the quoted rules (below, at or above the GM's "native" level).
Blue
American Steele: 50 BS/Inv
Nightfall: 50 DDD
Sable Slayer: 50 DM/Rgn
Fortune's Shadow: 50 Dark/Psi
WinterStrike: 47 Ice/Dev
Quantum Well: 43 Inv/EM
Twilit Destiny: 43 MA/DA
Red
Shadowslip: 50 DDC
Final Rest: 50 MA/Rgn
Abyssal Frost: 50 Ice/Dark
Golden Ember: 50 SM/FA
Here is what GM code does:
If you are a higher level than the Giant Monster, you will do damage to it as if you were its level.
If you are a lower level than the Giant Monster, you will do damage to it as if it was one level lower than you.
A Giant Monster does damage to you as if it were your level.
So would you damage it as if you were a level higher then it?