Well, technically speaking, if you've got a nimble enough character, you could probably sever what passes as the Giant's Popliteal Artery (Major artery past the knee) with a stabbing weapon, causing him to eventually bleed out. Or sever his Achille's tendon, making his calf muscle make an impression of one of those old roll-up window blinds and severely affecting his stability. Especially if you manage to get both. And then you could go for the neck, eyes, heart, etc.
It all depends, however, on how the game handles collision detection and if it can handle such script-heavy fights.
the achille's tendon thing I've actually seen IRL, one of the sickest things I think I'll ever see. I got a call to domestic disturbance and there was a guy who had the tendon slashed with a kitchen knife.... it gives me shivers just thinking about it.
I would think the best way to take down something taht big would be slashing those until he fell, then doing one of the "context sensitive" kills when he goes down. But I honestly don't care if a million slashes takes it down.