How in the *seven hells* did Martyn learn these spells? [BLU Lv.50~60 SPOILERS]

[Exodus-E]

Perhaps Martyn got a soulstone that wasn't empty unlike the ones we got, and one of the previous owners of it had fought sophia in the distant past

Doesn't seem likely.

The "Soul of the Blue Mage" crystals were made by Martyn himself, based off what he learned from the Whalaqee over the New World.
And even then, I'd assume it's a similar process to what SMNs do to get their Egis/Trances/Demi-Primals (i.e. facing/defeating their originals and/or attuning to their aether).

Maybe it's enough for him to just see or know about the attack to mimic the spell and he doesn't need to actually beat the Primal for it.

Which begs the question: how did he manage to get around their domains unseen/uninjured and with his mind intact...