@Saiyunaz you need to be 50 and above to get the highest tier of ARKS missions. I do believe you need to be the highest tier to get the keys but it been awhile since I had low level character.
Level 50 is only about 4.5mil exp. If you have some exp tickets you can probably skip alot of the lower levels and start doing lower tier daily ARKS mission for exp.
Only other tip I can really give you is to unlock Cofy's quest ASAP so you can switch to Hero since you have it unlocked on your main already, Scion classes are really easier to level compared to base classes with their higher stats, good weapon multipliers, don't need to worry about subclass, and don't need PA and Tech disc to get skills for the most part (unless you doing Rod Phantom - then you need those tech discs)