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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.

oh, my bad. my priest friend is full of it then