Hi, you can farm ap while level grinding. If your at max level already try Arcane Golem of Rage in Arcane Trace or the Ancient Dragon in Nostalgia Forest. EDIT: As he stated below you can also try Senilinea map. I would recomend the Meer King spot, just lure it a bit away from it's spawn spot into the group of mobs and with help of HP steal/Vamp Earrings etc. your good. You can also lure it out to reset it's HP and you can use bb axp(bought from Event Girl Yul for 180 vCoins) for an efficient ap farming.
I use BL as a main char, and the best way that I found for farming AP is with Senillinea bosses, since Arcane Golem have some mini-bosses around it with a lot more HP than before it's kinda annoying to farm AP there, Ancient Dragon have a higher attack than Arcane Golem/Senillinea bosses, I won't recommend that unless you have a higher HP steal/Max HP Steal per hit, I highly recommend Senillinea bosses