The first Alyssa coins are the fastest, unfortunately.

Main sources of Alyssa coins seem to be the Blaze Crowd (raid) orbs (souls) and crystal gacha (raid gacha). So what I did was quest until I was almost at level up (exp bar almost full), spend the AP+scrolls on easies to solo, spend BP on hard/death to leech orbs, go back to quest to level up and repeat ad nauseum.