When taking damage from toxic sources or some pyschic powers (notably the force weapon ability), it ignores armor and toughness. But does this also ignore True Grit? Do you still subtract your toughness from critical damage taken as the talent says? This is a different situation from dealing with normal toughness reduction of damage, and it makes a pretty big difference. I've got a sorcerer in my group who can easily one-shot enemy marines since his force weapon ignores true grit, while the rest of my group has to hack them down 1 hitpoint at a time. Applying true grit even against this damage would mean he'd 2 shot them instead.

How would you all handle it?

I can't remember the exact wording, but personally I would allow the use of True Grit against attacks that ignore Toughness, especially as the original version didn't care about soak (unless it had a Toughness prerequisite, but the way it worked was entirely independent of the value).

The True Grit description says "Whenever". So yeah, True Grit applies whenever the character goes into critical damage, even if it ignored toughness.

How about damage with felling quality. A Space marine with 8tb (unnatural +4) gets hit by felling 4 weapon. Lets say he's got 0 hit points, and takes 8 damage after reduction from tb. Does felling dmg reduces hit TB for the purpose of true grit, and he reduces hit crit dmg by four? Or it only counts for damage he takes, so he reduces crit by 8, and stays at +1 crit?

I don't think felling does anything for true grit al all. Felling does not do damage! It just negates unnatural toughness. If you have felling (4) and shoot against something with Unnatural Toughness (2), all the felling will do is negate the 2 points of damage reduction. It works exactly like Armour penetration except against Unnatural toughness.

Good thinkin' ! thx