If you've unlocked Legion invasions, once your alts hit 98 they can enter Legion, get their weapon, and do the Invasions to gain roughly 1 level for each invasion. Takes about 25 minutes per level. Invasions run for 6 hours, then come back after another 12 hours. XP remains high until you are 111.

I was running 4 teams and a few singles through each invasion at one point.