You said that brittle doesn't stack. But it does. Or at least it did, unless it got changed. His W and Ult both add a stack. In the entire wiki we have written than Ornn can reduce the Tenacity by up to 60%.
Tested it in practice tool. You never get 2 stacks of the debuff onto the enemy and you never deal more dmg even if you do use WRR (R2 should proc 2 stacks if it would stack, but only the R2 and only the 1 stack dmg gets triggered).
I am not sure how it works with tenacity and if it can still stack for this one. Haven't looked at the CC timers, but in terms of dmg it does not stack.