Mana Shield is an all or nothing skill. Having it loosely hanging like that is both MP inefficient and pointless, because it will drain your MP and not even help protect your HP at all.
Nen Masters have their plethora of buff attacks that make them near god-tier PvE, played right, they're also amazing PvP. One extra skill won't save you much in the long run, and you're much better off spamming X with your buffs activated than spamming skills like most classes in PvE. Coupled with Nen Cannon, Doppelganger, and Nen Guard, there's not much reason for MORE attack skills. In PvP, all Fighters just need the shoulder bump loop. A combo in PvP would be shoulder bump loop -> Suplex (Thunder). Suplex Thunder deals tons of damage already. OTG Energy Shield after Suplex to follow up deals even more. Doppelganger Blast honestly pales in comparison to Suplex Thunder.