Greek myth does not say how Thetis did so, only that she ensured he lived agian as a immortal upon Leuke.
It was not Thetis's heel that was vulnerable but that of her mortal son Achilles, this may have come about because she was called Argyropeza (silver-footed).
Thetis gave prophesy that she would have a son greater than his father; Zeus and Poseidon did not pursue her afterwards but made it so she would have a son by a mortal. That son was Achilles.
Achilles' Heel means the one weakness in an otherwise invincible thing. Achilles was the half-mortal son of the nymph Thetis, who dipped him in the River Styx to make him immortal. Unfortunately, she missed his heel, which she held; it remained mortal. Paris shot Achilles in the heel, presumably with a poisoned arrow, thus killing him where no other man could.
He was the son of Zeus and therefore half-immortal.
Yes, Achilles was the son of Peleus (mortal) and Thetis (divine). (Hercules) Heracles was the son of Zeus (divine) and Alcmene (human).
If by demigod you mean a mortal who had an immortal for a parent, there was Heracles, son of Zeus; Perseus, son of Zeus; Orpheus, son of the Muse Calliope; Achilles, son of the sea nymph Thetis; Deucalion, son of the Titan Prometheus; Phaethon, son of Helios; Asclepius, son of Apollo; Minos, Sarpedon, and Radamanthus, sons of Zeus; Tantalus, son of Zeus; Helen, daughter of Zeus; and Pollux, son of Zeus.
Thetis does not promise anything, but Zeus does promise Thetis that her son Achilles will be honored.
That Thetis will know how her son is dishonored and seek to make things right for Achilles by going to Zeus.
Achilles
Achilles.
No, Achilles is the son of Peleus and Thetis.
Thetis largely helped her son Achilles achive his wish of glory.