It would seem that Achilles being dipped in the River refers to an old Smithy tale of Quenching: dipping the red-hot iron/carbon meat, aka steel, in water or oil to harden it. It is known Achilles wore a suit (of apparently steel) armor. The "weak spot" would be where the smithy held the armor during the quenching process.
Regarding "Indo-European" origins of metal-working, I would highly recommend this paper, found at academia.edu:
Tvaṣṭr, Meluhha of Bhāratam Janam (Rigveda) is Tuisto, divine ancestor of Germanic peoples (Tacitus), legacy of Proto-Indo-Aryan superstrate & Mitanni Treaty
This can be seen as a pilgrimage, a journey of Meluhha artisans/sea-faring merchants in Eurasia, during the Bronze Age, along the Maritime Tin Road from Hanoi, Vietnam to Haifa, Israel.
This journey also explains why many Proto-Indo-Aryan words like those present in Mitanni Treaty occur on Indus Script Corpora which is a veritable catalogus catalogurm of metalwork by metalcasters in the tradition of Tvaṣṭr , Tuisto...Tvaṣtṛi is the Vulcan of the Hindu mythology. He had a son named Triśiras...Meluhha of the Indian sprachbund, is the spoken form of chandas, the prosody of Rigveda. Tvaṣṭr is the metal artificer par excellence, who forges the vajra (thunderbolt) weapon for Indra, a narrative celebrated in exquisite metaphors of Rigvedic chandas..."King Shaushtatar (ruled c. 1430) extended the boundaries of Mitanni through the conquest of Alalakh, Nuzi, Assur, and Kizzuwatna. Egypt, under Tuthmosis III (1479-1425 BCE), defeated the Mitanni at Aleppo after a long period of contention over control of the region of Syria. Later Egyptian dynasties entered into pacts and treaties with Mitanni and the daughter of the Mitanni King Tushratta, the princess Taduhepa, was given in marriage to Amenhotep III (1391-1353 BCE) as part of a treaty which balanced power between the two nations.This treaty was put to the test during a power struggle in Washukanni between Tushratta and a relative of the previous king, Shuttarna, known as Artatama II.
I am of the opinion that "Tadukhepa" is the same name as the Russian "Dudieva" ("khepa" is pronounced Eva or Eve) and she is none other than "Nefertiti." Further, King Shashtatar reminds strongly of Esua. Further, it seems clear chronology must be re-examined stringently: is this king not a Tartar? "Scratch a Russian and underneath you'll find a Tartar."