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Priests do NOT get a 150% boost if they are by themselves. it's 110% if they're alone. So no, they don't get it whenever they want. Even in a party of two... the total best exp possible that they can get is 110% of the original... it wont go higher (that's after adding in the party bonus exp). It's only major boost if there's a party, meaning everyone in the party gets the benefit not just the priest.

Ok, priests gets the boost all the time, improved when partied... Yes, it's improved. He can kill too. And it's valuable for getting a party easily.

Why not make it still work on bosses but only have a lower effect? Consider that it's worse at training because you have to actually spend an extra attack for it to work.