The signature move of a primal is the one that auto-kills unless certain requirements are met. For Titan, it's the move he does immediately after Heart phase ends. For Garuda, it's the one that's not blocked by the pillars. For Ifrit, it's what he uses after the nails.
As far as it being magic-based, the damage gets absorbed by Manaward and can't be parried, so it must be magic damage.
Also, if you want to test whether an ability is magic damage or physical damage, activating both Manaward and Manawall isn't the way to do it, because you see it reduced to 0 damage and don't necessarily know which ability negated it.