I'd recommend giving it enough Guard and armor that it can survive at least one round of four average knights whaling on it. (Average knights deal around 5.5 damage.) As is, it would probably die from 3-4 regular attacks. Maybe give it more guard and a point of armor? (Could have scales or slippery skin). 

Nix

VIG 16, CLA 16, SPI 7, 12GD

A1 (Scaled hide)

That would allow it to survive five regular attacks on average. You could also give it some form of conditional immortality, like can only be harmed by someone who can out-fiddle him, or can only be harmed by his betrothed. As for damage, it looks good to me. 2d10 has almost 50% odds to critical wound a knight if it bolsters, which feels suitably climactic for a boss fight.