I'm going to put what I said as a comment as an answer.
The answer really depends on what set and format you're playing. I'll be talking about two formats, RU and National Dex. In RU, Reuniclus has two sets in can use: Defensive Calm Mind and Assault Vest. Here are the two sets for reference:
Reuniclus @ Leftovers
Ability: Magic Guard
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Calm Mind
- Psyshock
- Focus Blast
- Recover
Reuniclus @ Assault Vest
Ability: Regenerator
EVs: 248 HP / 24 SpA / 236 SpD
Modest Nature
- Psyshock
- Future Sight
- Focus Blast / Knock Off
- Shadow Ball / Knock Off
On the Defensive Calm Mind set, you can use Shadow Ball to deal super effective to opposing psychic and ghost types, but then Reuniclus misses out on hitting dark and steel types. On the Assault Vest set, Reuniclus runs Focus Blast alongside Shadow Ball so you can be able to hit psychic, ghost, dark and steel types. Focus Blast can be replaced with Knock Off which is used as a utility move that cripples Pokemon switching if you have teammates that can sufficiently defeat Focus Blast targets.
In National Dex, this is the set Reuniclus uses:
Reuniclus @ Fightinium Z / Electrium Z
Ability: Magic Guard
EVs: 252 HP / 212 Def / 44 Spe
Bold Nature
- Calm Mind
- Psyshock / Stored Power
- Focus Blast / Thunder
- Recover
Usually, Reuniclus uses Focus Blast as the coverage move for this set because with Fightinium Z, you have a one-time fighting move that will not miss. Thunder with Electrium Z can be used alongside Stored Power, with Electrium Z Thunder being able to be a good general nuke and being able to hit Toxapex hard. However, by using Electrium Z + Thunder, Reuniclus won't really be able to deal with dark types, which can cause you to become Pursuit trapped more easily.
tl;dr It comes down to what format and what set you use. Focus Blast is the better option usually if running a Calm Mind set, as it allows you to hit dark and steel types. Shadow Ball is an option to use on Calm Mind sets, but you miss out on dealing super effective damage to dark and steel types. On Assault Vest sets, you use Focus Blast alongside Shadow Ball. There are a few coverage options, but it depends on what format you play.