Magic Guard w/ Leftovers/Life Orb
Modest/Quiet w/ 252 Sp Attack, 252 HP, 6 Defense/Sp Defense
Psyshock/Psychic
Shadow Ball/Night Shade
Focus Blast
Energy Ball/Thunder/Trick Room/Endeavor/Pain Split
Partners (for Double/Triple battles)
Trick Room-users if Reuniclus hasn't already have it.
Reflect-users (Recommending a non-Psychic type)
Countering
High Sp Defense with Bug/Dark/Ghost-type moves like Bite Flareon/Gyarados, Spiritomb, X-scissor Escavaleir, or Shadow Ball Togekiss.
First off, Shadow Ball will always do more damage to whatever Reuniclus is hitting over Night Shade, there is no good reason to use Night Shade over Shadow Ball on Reuniclus. Secondly, an all out attacker set is easy to wall, as switch ins other the Ferrothorn will all be faster than it, so it should be running Trick Room on this sort of set (which, to be fair, you did put in the about five slashed options, but it really should take precedence.) Partners for it, I agree on the Trick Room users, but Reuniclus will be needing Light Screen just as much if not a CM set, so it's probably not worth mentioning either, as Reuniclus' bulk generally allows it to survive a few hits anyhow.
Moving onto the counters, most of these are terrible ideas, high special defense won't matter if Psyshock is used (which it commonly is), and just because something has a super effective move doesn't mean that it'll be effective at all. Here are some calculaitons for the things you listed (the sets for Reuniclus I'm listing are the most commonly used sets):
252+ SpA Life Orb Reuniclus Psyshock vs. 252 HP / 4 Def Flareon: 273-321 (81.73 - 96.1%) -- guaranteed OHKO after Stealth Rock (This is Offensive TR vs SDef Flareon)
0 SpA Reuniclus Psyshock vs. 252 HP / 4 Def Flareon: 157-186 (47 - 55.68%) -- guaranteed 2HKO after Stealth Rock (This is CM set vs SDef Flareon)
0- Atk Flareon Bite vs. 192 HP / 64 Def Reuniclus: 114-136 (27.87 - 33.25%) -- guaranteed 4HKO (Same sets in reverse, SDef Flareon against OTR Reuniclus)
0 SpA Flareon Flamethrower vs. 252 HP / 4 SpD Reuniclus: 112-133 (26.41 - 31.36%) -- possible 4HKO (SDef Flareon vs CM Reuniclus, Bite does even less)
So as we can see, Flareon can't switch in on Reuniclus at all, being OHKO'd or 2HKO'd even when Reuniclus has no boosts, and Flareon can't even 4HKO it in most cases in return (as it's highly unlikely to survive more than 1-2 turns itself), and Bite is a terrible option. Moving on to the next counter:
252+ SpA Life Orb Reuniclus Psyshock vs. 56 HP / 0 Def Gyarados: 220-261 (63.76 - 75.65%) -- 6.25% chance to OHKO after Stealth Rock (OTR vs SubDD Gyarados)
252+ SpA Life Orb Reuniclus Psyshock vs. 252 HP / 252+ Def Gyarados: 153-181 (38.83 - 45.93%) -- 92.58% chance to 2HKO after Stealth Rock (OTR vs Resttalk Gyarados)
0 SpA Reuniclus Psyshock vs. 56 HP / 0 Def Gyarados: 127-151 (36.81 - 43.76%) -- 58.98% chance to 2HKO after Stealth Rock (+0 CM vs SubDD)
+1 0 SpA Reuniclus Psychic vs. 56 HP / 0 SpD Gyarados: 177-208 (51.3 - 60.28%) -- guaranteed 2HKO after Stealth Rock (+1 CM vs SubDD)
0 SpA Reuniclus Psychic vs. 252 HP / 4 SpD Gyarados: 118-139 (29.94 - 35.27%) -- guaranteed 3HKO after Stealth Rock (+0 CM vs Resttalk)
+1 0 SpA Reuniclus Psychic vs. 252 HP / 4 SpD Gyarados: 175-207 (44.41 - 52.53%) -- guaranteed 2HKO after Stealth Rock (+1 CM vs Resttalk)
248+ Atk Gyarados Bounce vs. 192 HP / 64 Def Reuniclus: 174-205 (42.54 - 50.12%) -- 0.39% chance to 2HKO (+0 SubDD vs OTR, Bite does less)
0 Atk Gyarados Bite vs. 192 HP / 64 Def Reuniclus: 124-146 (30.31 - 35.69%) -- 33.72% chance to 3HKO (Resttalk vs OTR)
248+ Atk Gyarados Bounce vs. 252 HP / 252+ Def Reuniclus: 127-151 (29.95 - 35.61%) -- guaranteed 4HKO (+0 SubDD vs CM, Bite does less)
0 Atk Gyarados Waterfall vs. 252 HP / 252+ Def Reuniclus: 91-108 (21.46 - 25.47%) -- possible 5HKO (Resttalk vs CM, Bite does less)
I chose these two sets for Gyarados as they are actually the ones that fared best against Reuniclus' attacks, and it is still pretty sad. Bite is not usable on Gyarados except in one instance, where sacrificing other coverage for Bite would be extremely detrimental, and in all other cases there was a move that did more than Bite. Gyarados is not a good counter due to the fact that Reuniclus generally has a good chance of 2HKOing, whereas Gyarados has a very minimalistic chance to 2HKO, more likely a 3HKO, so Reuniclus would end up KOing Gyarados before it could finish off Reuniclus. I had the SubDD set for Gyara at +0 as it is supposed to be countering Reuniclus, meaning it is likely trying to switch in on Reuniclus and thus hasn't had opportunity to set up yet. And if it takes said opportunity, it is in danger of being KO'd that same turn or the next turn, meaning it will only get one hit in, which will not ever KO Reuniclus unless it crits. Next counter:
252+ SpA Life Orb Reuniclus Shadow Ball vs. 172 HP / 0 SpD Spiritomb: 114-135 (40.14 - 47.53%) -- 49.22% chance to 2HKO after Stealth Rock (OTR vs All-Out Attacker)
252+ SpA Life Orb Reuniclus Shadow Ball vs. 252 HP / 4 SpD Spiritomb: 113-134 (37.17 - 44.07%) -- guaranteed 3HKO after Stealth Rock (OTR vs +0 CM)
252+ SpA Life Orb Reuniclus Shadow Ball vs. 252 HP / 64 SpD Spiritomb: 108-127 (35.52 - 41.77%) -- guaranteed 3HKO after Stealth Rock (OTR vs Defensive)
252+ Atk Choice Band Spiritomb Sucker Punch vs. 192 HP / 64 Def Reuniclus: 398-470 (97.31 - 114.91%) -- 81.25% chance to OHKO (All-Out Attacker vs OTR, other moves will 2HKO)
0 SpA Spiritomb Dark Pulse vs. 192 HP / 0 SpD Reuniclus: 186-218 (45.47 - 53.3%) -- 32.03% chance to 2HKO (+0 CM vs OTR)
0 Atk Spiritomb Sucker Punch vs. 192 HP / 64 Def Reuniclus: 188-224 (45.96 - 54.76%) -- 61.33% chance to 2HKO (Defensive vs OTR, other moves will 4HKO)
252+ Atk Choice Band Spiritomb Sucker Punch vs. 252 HP / 252+ Def Reuniclus: 294-348 (69.33 - 82.07%) -- guaranteed 2HKO (CB vs CM, other moves 3HKO)
0 SpA Spiritomb Dark Pulse vs. 252 HP / 4 SpD Reuniclus: 186-218 (43.86 - 51.41%) -- guaranteed 3HKO (CM vs CM)
0 Atk Spiritomb Sucker Punch vs. 252 HP / 252+ Def Reuniclus: 140-168 (33.01 - 39.62%) -- 19.04% chance to 3HKO (Defensive vs CM, other moves will 5HKO at best)
OK, Spiritomb may be one of the few things you listed that has a possibility of countering Reuniclus. A Reuniclus CM set without Shadow Ball cannot hit Spiritomb (or Sableye for that matter), thus they can be considered counters. Though the problem with Spiritomb is that it depends on Sucker Punch for its reliable KOing, meaning the CM set can just Recover off previous damage or damage caused by a different move from Spiritomb, none of which would KO Reuniclus. OTR would likely have to switch out as it cant just spam Trick Room forever to avoid Sucker Punch and it has no way to heal off the damage caused by Spiritomb. Onto Escavalier:
252+ SpA Life Orb Reuniclus Focus Blast vs. 248 HP / 8 SpD Escavalier: 173-204 (50.43 - 59.47%) -- guaranteed 2HKO after Stealth Rock (OTR vs CB)
252+ SpA Life Orb Reuniclus Focus Blast vs. 248 HP / 252+ SpD Escavalier: 126-149 (36.73 - 43.44%) -- guaranteed 3HKO after Stealth Rock (OTR vs SDEf Esca)
0 SpA Reuniclus Focus Blast vs. 248 HP / 8 SpD Escavalier: 100-118 (29.15 - 34.4%) -- 100% chance to 3HKO after Stealth Rock (+0 CM vs CB)
+1 0 SpA Reuniclus Focus Blast vs. 248 HP / 8 SpD Escavalier: 149-176 (43.44 - 51.31%) -- 98.83% chance to 2HKO after Stealth Rock (+1 CM vs CB)
0 SpA Reuniclus Focus Blast vs. 248 HP / 252+ SpD Escavalier: 73-87 (21.28 - 25.36%) -- possible 5HKO after Stealth Rock (+0 CM vs SDef)
+1 0 SpA Reuniclus Focus Blast vs. 248 HP / 252+ SpD Escavalier: 109-129 (31.77 - 37.6%) -- 98.12% chance to 3HKO after Stealth Rock (+1 CM vs SDef)
252+ Atk Choice Band Escavalier Megahorn vs. 192 HP / 64 Def Reuniclus: 774-912 (189.24 - 222.98%) -- guaranteed OHKO (CB vs OTR, X-Scissor does much less)
252+ Atk Choice Band Escavalier Megahorn vs. 252 HP / 252+ Def Reuniclus: 576-678 (135.84 - 159.9%) -- guaranteed OHKO (CB vs CM, X-Scissor doesn't even OHKO the majority of the time)
8 Atk Escavalier Megahorn vs. 192 HP / 64 Def Reuniclus: 392-464 (95.84 - 113.44%) -- 75% chance to OHKO (SDef vs OTR, X-Scissor 2HKOs)
8 Atk Escavalier Megahorn vs. 252 HP / 252+ Def Reuniclus: 290-344 (68.39 - 81.13%) -- guaranteed 2HKO (SDef vs CM, X-Scissor 3HKOs)
Escavalier does have a good chance of countering Reuniclus. CB has to be wary of switching in as it could be taken out by the OTR set before it even gets a chance to attack, but otherwise will have no trouble with Reuniclus. SDef Escavalier has nothing to fear from Reuniclus and can kill it at its leisure. The main problems with Escavalier was the move you suggested using with it, X-Scissor. Albeit more reliable than Megahorn, it's damage is pretty pitiful in comparison, having trouble 2HKOing Reuniclus even with Choice Band. Megahorn is the superior choice, and X-Scissor should not be used. Another thing to note is that Escavalier is outclassed severely in OU by Scizor, who fulfills the same role and does it better, meaning Scizor would be a better choice to counter Reuniclus. So, even though Escavalier counters Reuniclus, Scizor does it better. Moving onto Togekiss:
252+ SpA Life Orb Reuniclus Psyshock vs. 252 HP / 4 Def Togekiss: 188-224 (50.26 - 59.89%) -- guaranteed 2HKO after Stealth Rock (OTR vs Paraflinch)
0 SpA Reuniclus Psyshock vs. 252 HP / 4 Def Togekiss: 109-129 (29.14 - 34.49%) -- guaranteed 3HKO after Stealth Rock (+0 CM vs Paraflinch)
+1 0 SpA Reuniclus Psyshock vs. 252 HP / 4 Def Togekiss: 162-192 (43.31 - 51.33%) -- guaranteed 2HKO after Stealth Rock (+1 CM vs Paraflinch)
0 SpA Togekiss Shadow Ball vs. 192 HP / 0 SpD Reuniclus: 156-184 (38.14 - 44.98%) -- guaranteed 3HKO (Paraflinch vs OTR)
0 SpA Togekiss Shadow Ball vs. 252 HP / 4 SpD Reuniclus: 154-182 (36.32 - 42.92%) -- 97.85% chance to 3HKO (Paraflinch vs +0 CM)
0 SpA Togekiss Shadow Ball vs. +1 252 HP / 4 SpD Reuniclus: 102-122 (24.05 - 28.77%) -- possible 5HKO (Paraflinch vs +1 CM)
K, Togekiss isn't that great of a counter either. OTR and +1 CM will beat Shadow Ball Togekiss in all cases, and +0 CM will beat it if it hits Togekiss on the switch in. If Togekiss 1) carries Shadow Ball at all and 2) if CM Reuniclus never uses Recover and 3) if CM Reuniclus doesn't use CM, then it has a chance of beating it. Otherwise, Togekiss gets beaten. Not a good counter.
So essentially, for you
tl;dr people, Reuniclus is only countered by two things that you listed, one of those being severely outclassed in the tier Reuniclus is in, so not a very good counters list at all really. The high special defense users have trouble with Psyshock, with even Chansey and Blissey getting 2-3HKO'd, and not able to do much in return as Magic Guard protects Reuniclus from debilitating status, and Seismic Toss can usually only 4HKO Reuniclus at best.