The best way, GNB 70

[Jandor]

Armory bonus, IIRC, is the bonus XP you get whenever you're leveling something lower than your highest level job.

So for example, if you have a level 70 Summoner, anything below that level gets extra XP.

In SB, it's anything below 60 gets a 100% bonus, and anything above 60 but below 70 gets a 50% bonus. In ShB this will be adjusted to take the new level cap into effect.

Quickest way to 70 will probably be getting to level 61 with roulette's then going to the Heaven on High tower in the Ruby Sea.
Roulette queues should be fast enough for GNB, it's still a tank after all, but I think trying to go through the SB leveling dungeons in order will take quite a while, can see the healer section of the duty finder being pretty empty as healer mains just head straight into the new expansion.