Level up one class or several from the beginning.

[Garlyle]

I thought the extra 50% bonus was only given once you reach 60, not 50??

Nope. The Armoury bonus is entirely tied to whether the class being lower levelled than your highest. The amount of bonus EXP received is then dependant on what level your class in question is at - classes below 50 get +100%; classes from 50 onwards get +50%.