>>17948150Quiet is detrimental if you're not running a Trick Room team; go Modest.
Magic Guard gives Reuniclus the fantastic opportunity to use a Life Orb without recoil damage which means using Choice Specs would be shooting yourself in the foot.
To maximize bulk while being able to deal with all threats I go 252 HP / 252 SpA (to make best use of Life Orb) / 4 Def
I run mine as a Trick Room-setting lead so I go for maximum coverage with Psyshock, Shadow Ball, and Focus Blast (with Trick Room being the fourth slot).
For an all-out attacker, Energy Ball in that fourth slot would round out coverage well. However, Recover would help it stay in the game a lot longer and Recover coupled with Calm Mind is incredibly effective. Also, Psyshock is definitely better than Psychic on Reuniclus as it allows it to avoid getting walled.
With all that said, I do think it's worth considering keeping Trick Room. In a singles team, you can effectively run Trick Room with only two pokemon (one of which being Reuniclus) that actually need its support. I personally lead with Reuniclus and then have Mega Mawile as an alternative, but there are other great alternatives like Conkeldurr and Azumarill who stand on their own even without Trick Room and kick even more ass while under its effect.