New Player

[MithrasInvictus]

Another question.

Say I level an arcanist to level 30, and whatever the summoners other subclass is to 15, and then I level the summoner to level 50.

Would I have access to a level 46 arcanist spell such as miasma II or would I need to level my arcanist to level 46 as well ?

It will always be your ACN getting the experience points, the SMN is just a modifier for the ACN class which is equipped to your character like a weapon or piece of armour.