You can’t “hit someone else”* because of the stipulations in contingency. RAI I’m sure they don’t want you sucking the life from your left buttcheek to close up the mortal wound in your gut, but based on what they wrote, I haven’t found a solid contradiction to this plan in the rules-as-written.
Also you can deal damage to something that is fully damaged. There’s even a specific rule for “Damage at 0 Hit Points.” under the Death Saving Throws section.