MY job

[velric42]

I want to become a summoner so should I take my Arcanist class to lvl 30 then switch to the other class that I need?

[Sessurea]

yes
/10 char

[Niwashi]

Take your Arcanist to anywhere between level 10 (where you unlock the ability to add additional classes) and 30 (when you'll be wanting to unlock the Summoner job). It doesn't matter when within that 10-30 range you choose to take a break from Arcanist and level Thaumaturge to at least level 15, but I would suggest doing it before you get much past 30 on Arcanist.

[Shmunkster]

As Niwashi says, you can do it at any point, after you get to join the other guild (thurma), but before you get to 30...as then you can cruise straight into Summoner life!