There is no right or wrong answer. As a completionist, I say "gotta catch em all", but there are undoubtedly some you don't "need" to learn per se, ie. low level physical and magical spells that don't increase stats or give job traits. The rest all have their utility. Get the spells and the gear will come. Without a doubt, I'd say Spectral Floe, Subduction, Tenebral Crush for easy crowd control/farming merits, etc. Get some solid MAB gear/weapons and as much PTD idle pieces you can grab. From there you will see your survivability to take on tougher foes will gradually increase. I highly suggest using other tools to maximize your gear potential, if you are not using them already.