Actually you get a 50% exp bonus to any DoW/DoM that is lower than your highest level class. Doesn't have to be 50.
Just do a low level roulette everyday after you get to lvl 16. It doesn't take long doing that to level.
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Levelling ACN is a walk in the park, at least it was for me.
You need to abuse the fact you have a personal tank; Carby. Get hunting logs done, and fates. Shouldnt take long.
Daily low level roulette and guildhest roulette are two biggest sources of exp. I suggest to do them on ACN as it counts for dps role and has much longer queue time.
If you are going to stick with arcanist - getting lv26 THM for swiftcast is a must because dead fairy may cause a wipe. CNJ lv34 for ACN is quite useless: x-classed stoneskin is only 10% instead of 18%, so pretty much every time it will be cast by WHM, not SCH.
For WHM getting THM lv26 is optional - swiftcast is required to revive a bad dps, who likely will die again to the same mistake and only waste your mana and time, but surecast (lv8) is quite useful for casting medica 2. ACN lv34 is important for WHM for Eye for an Eye skill - reduces the damage tanker takes by 10% for 30s, which helps a lot in threat and mana management.
WHM is more viable as main healer and solo healer, SCH is more viable as tanker healer and bad players healer (better mana pool and quicker heals).