Ok, let's explore the Grey Knight in Terminator Armor. We'll even use the exact stats from the book.
So you've managed to land a hit with a flame weapon on a Grey knight and his innate field hasn't ignored it completely (1 in >3 chance).
Fate Points 2
As I've already talked about on average it's going to take a while to kill him outright. At 3 Wounds every ten rounds that's 90 rounds to get to Critical Damage, and they have True Grit, so 1 wound no matter what after that. So 2 wounds every 10 rounds, reset at -5 with a fate point so it's effectively 150 rounds before the Grey Knight Dies. So death from damage isn't feasible, and fatigue will happen much quicker.
Being on Fire has the wonderfull side effect of a Willpower roll each round, and Grey Knights have resistance heat, so honestly they have a 2 in 3 chance of succeeding each round. This drops back down to 56% (1 in >2) once a level of fatigue is recived)
Now on to the fatigue. The Grey Knight can take 8 Levels of Fatigue before passing out for 2 minutes. He has 2 fate points and won't be needing them for wounds as we can see above, so we can safely say the Grey Knight has 24 rounds to succed at an agility test with a 0 modifier. Painfull, unless they can find some water in which case I'd call the test a bonus of at least +10, probably +20, or automatic success if they just submerge themselves. But, for whatever reason you're fighting on top of a mountain, with no usable environment around. No walls to knock down and create dust, no sand to roll around in, no water nearby, no plantlife, just empty open space.
That's 16 rounds (failed willpower checks) of Storm bolter Shells firing at you with a 45% chance of success (35 after he's on fire (1 in >3). And you're not well enough geared to penetrate his armor so are having to pull some Fire schenanigans. Sure you're in defensive stance, but he's shooting you. Oh you can run, but it's likely he'll chase after you and HammerHand instead which is even worse.
So 6 hits with a storm Bolter on Semi auto. You've not invested in Meltas or Plasma weaponry, so you're likely in Carapace. Even Heavy Carapace. Your Toughness bonus is 4 or 5, let's go with 5. 6 hits is 12 hits, with Tearing and Storm, that's going to be 7 damage rolled per hit. Very unlikely he's going to jam. so 7+9 damage per hit (16) times 12 hits. You have AP 6, so you reduce damage by 7 each time you're hit.
How many wounds did you have again? More than 108?
Yo dawg, I don't think this is a scenario that would work.