NOTE: All base stats WERE CORRECTLY EDITTED IN THE CALC. I simply typed in the correct base stats, then EVed the monsters as I saw fit.
I believe Reuniclus should be dropped from A rank Viability to perhaps B+, and actually be replaced by his little form DUOSION! There is no arguing that Reuniclus getting an extra OOMPH didn't make it usable in a big way. Much like Suicune, Reuniclus is an improved force that uses it's tiny UU boost to make the best of some great abilities and really rise in usability.
Reuniclus hp115/att70/def80/spA130/spD90/spe35 (UU, UU boosted)
Duosion DOE hp85/att60/def70/spA145/spD80/Spe50 (NFE, PU boosted)
Duosion effectively gets to compare itself to it's evolved form (their abilities are all the same too, gr8!), so the basis of my argument will simply be some calcs (and scenarios), and me defending Duosion's lack of focus miss.
First though, let me defend Duosion from the obvious "Duosion gets no focus blast, how can it be better!?" argument. Focus blast is the only move Duosion really needs from its evolution... HOWEVER an 8 PP 110 base move, with 70% acc and only a 10% proc rate (stat dropper) FIGHTING can be replaced by a 16 pp 110 base move, with 70% acc and a gnarly 30% proc rate (STATUS) electric type. Thunder has you covered vs many threats, has higher Power Points (since tier shift can be stally it CAN END UP MATTERING) and Thunder's Status, Paralysis, doesn't go away if the foe switches. It can effectively cripple unsuspecting foes trying to come out knowing your strongest fighting move possible is Hidden Power, such as a Tyrantrum, eating it's Lum or ruining it's chances of getting fast from dances!
Possibly useful moves Duosion cannot learn that Reuniclus CAN: Grass Knot (Duosion DOES learn Energy ball though, 90 base, 16 pp, 10% drop spD. Grass Knot isn't really needed, hits suicune 10 base harder, and you dont even wanna fight suicune. You hit Quag harder with energyball than with grass knot, also.)
Other moves Duosion cannot learn that Reuniclus CAN which are physical: Superpower, Drain Punch, Fire Punch, Thunder Punch, Ice Punch, knock off, Power Up Punch.
With that out of the way let's see some delicious Calcs!
First let's see POWER. Duosion also gets life orb/magic guard, and a massive 145 special attack stat to launch off of. If you decided to run Duosion in the same way as a Reuniclus and have a little less staying power (I wouldn't suggest this in tier shift but netting OHKOs on certain foes using that 145 base special attack is a HUGE help and is pretty tricky to unsuspecting switch happy opponents)
252+ SpA Life Orb Reuniclus Psychic vs. 0 HP / 4 SpD Tyrantrum: 285-335 (87.6 - 103%) -- 18.8% chance to OHKO
252+ SpA Life Orb Duosion Psychic vs. 0 HP / 4 SpD Tyrantrum: 308-363 (94.7 - 111.6%) -- 68.8% chance to OHKO
252+ SpA Life Orb Reuniclus Psyshock vs. 252 HP / 252+ Def Blissey: 312-368 (43.6 - 51.5%) -- guaranteed 3HKO after Leftovers recovery
252+ SpA Life Orb Duosion Psyshock vs. 252 HP / 252+ Def Blissey: 339-399 (47.4 - 55.8%) -- 22.3% chance to 2HKO after Leftovers recovery
252+ SpA Life Orb Reuniclus Thunder vs. 248 HP / 36 SpD Alomomola: 520-614 (94 - 111%) -- 62.5% chance to OHKO
252+ SpA Life Orb Duosion Thunder vs. 248 HP / 36 SpD Alomomola: 564-665 (101.9 - 120.2%) -- guaranteed OHKO
NOW for the GOOD stuff. We can use REGENERATOR/Eviolite and make this 145 base special attacker into a sustainable Mind Cannon who can keep coming back in and blasting your brains out with super cellular psychic shit shows!!! THIS is why I believe Duosion really belongs as an A RANK THREAT!! This particular combo lets you have offensive power over Reuniclus, longevity over it in your higher defenses, and even encourages that you switch out and avoid knock offs by rewarding each switch with 33% of your HP back!
NOTE: I made EV investment so both defenses would end up the same, and then threw the rest into HP so that hits would literally be absorbed equally specially and physically, that's why each has 80 def EV and 0 sp defense, so their real stats are (reuni) def216/spD216 and (duosion) def196/spD196
252+ Atk Life Orb Strong Jaw Tyrantrum Crunch vs. 176 HP / 80 Def Reuniclus: 411-486 (99 - 117.1%) -- 93.8% chance to OHKO
252+ SpA Reuniclus Focus Blast vs. 0 HP / 4 SpD Tyrantrum: 376-444 (115.6 - 136.6%) -- guaranteed OHKO (70% acc)
252 Atk Life Orb Strong Jaw Tyrantrum Crunch vs. 176 HP / 80 Def Eviolite Duosion: 276-325 (77.7 - 91.5%) -- guaranteed 2HKO
252+ SpA Duosion Psychic vs. 0 HP / 4 SpD Tyrantrum: 237-279 (72.9 - 85.8%) -- guaranteed 2HKO
Strong Jaw is ONLY used here because it'll hit the hardest, makes the calc more dramatic.
Now here Focus blast comes in handy, OFC! But Reuniclus is slower and ALMOST guaranteed to die to a single hit (as if anyone strong jaw crunches). Still, We're starting to see the defensive nature of Duosion already, surviving a strong jaw hit and CRIPPLING Tyrantrum, then switching away to simply regenerate 33% as you go, leaving behind a mangled addition to fossil fuels, and if he danced you simply bring out a rough skin/rocky helmet/iron barb monster and bye bye beastie, no damage done to your dangerous duosion dude!
+3 252 SpA Manaphy Scald vs. 176 HP / 0 SpD Reuniclus: 312-367 (77 - 90.6%) -- 25% chance to OHKO after Stealth Rock(30% burn for 12% also, since this is REGEN, making the chance for Reuniclus to die even higher, but he'd still get to attack)
252+ SpA Reuniclus Energy Ball vs. 0 HP / 0 SpD Manaphy: 210-248 (61.5 - 72.7%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
OR (ALTHOUGH chances of Reuniclus running thunder is much lower than Duosion, since Focus Blast is likely to take that spot) 252+ SpA Reuniclus Thunder vs. 0 HP / 0 SpD Manaphy: 256-302 (75 - 88.5%) -- 6.3% chance to OHKO after Stealth Rock
+3 252 SpA Manaphy Scald vs. 176 HP / 0 SpD Eviolite Duosion: 219-258 (61.6 - 72.6%) -- guaranteed 2HKO after Stealth Rock (cannot be one hit, even if burned)
252+ SpA Duosion Energy Ball vs. 0 HP / 0 SpD Manaphy: 234-276 (68.6 - 80.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
OR (much more likely Duosion runs Thunder, since it doesn't have access to focus blast!)
252+ SpA Duosion Thunder vs. 0 HP / 0 SpD Manaphy: 286-338 (83.8 - 99.1%) -- 75% chance to OHKO after Stealth Rock
Here we start to see some of the differences between the two, and the advantage of THUNDER vs FOCUS BLAST, in that Manaphy also likes RAIN, and sometimes you will have 100% acc Thunders! We also see that Reuniclus could be one hit, and WOULD be out sped, making it necessary for Reuniclus to flee where Duosion simply launches off an attack and more than 50% of the time (due to acc and odds but its str8 up 75% in rain) ends up netting a KO!
+1 252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 176 HP / 80 Def Reuniclus: 438-516 (105.5 - 124.3%) -- guaranteed OHKO
252+ SpA Reuniclus Psychic vs. 0 HP / 0 SpD Mega Charizard X: 180-213 (60.6 - 71.7%) -- guaranteed 2HKO after Stealth Rock
+1 252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 176 HP / 80 Def Eviolite Duosion: 306-361 (86.1 - 101.6%) -- 12.5% chance to OHKO
252+ SpA Duosion Psychic vs. 0 HP / 0 SpD Mega Charizard X: 201-237 (67.6 - 79.7%) -- 37.5% chance to OHKO after Stealth Rock
Here we have a real difference, without rocks on your side, with a 37.5% chance to end the opposing Charizard's sweep in one hit and 12.5% chance to die. Assuming you switch right in on it after the dance that would be a game changing 7/8 chance that Duosion gets a MORE THAN 1/3 chance to END the mega, and retreat to regen!
There are also GREAT calm mind/acid armor/recover sets that Duosion can do just as well as Reuniclus, IF NOT BETTER. However hanging around and setting up can end up in you taking a nasty Knock Off, and losing an eviolite if you're carrying one, though it's not entirely necessary to have eviolite considering how Duosion hits 15 special attack harder, and there is only a 30/10/10 difference in defenses it's ALMOST a fair trade even without evio, just to net OHKOs.
Duosion effectively comes out hits hard and gets the fuck away, it's a great way to go, and I'd strongly recommend it be placed in A RANK, and either taking Reuniclus' rank down a few steps, or placing directly above Reuniclus in A RANK (but I find Reuniclus inferior, honestly only shining in trick room settings due to Duosion's higher speed.)
Sure, Duosion doesn't have a Fighting move! Let's face it, it doesn't wanna take a hit from a dark type anyway and would still be able to signal beam it and leave for some extra damage over Hidden power. Over all, avoiding focus blast and running attacks like THUNDER, and Energy Ball for the Bulky water threats maybe beneficial even on Reuniclus at times, so I don't feel entirely hindered without Focus blast here, honestly.
That's all I've got. I really enjoy Tier shift, and I do not post often, though I have been around for quite some time and played competitively before I even joined smogon, being a shoddy battler and pokemon online battler, as well as VGC player long before I even decided the join the forums in order to vote on bans!
Hope to post more here later and I hope you accept my suggestions because I 'studied' a bit by playing through with a few variations and it takes a lot of time to find tier shift battles and play them! You can imagine tryin both Reuniclus and Duosion on multiple varities of teams took some time!