HI! Cool hail team, but as said before Reuniclus can give your team a lot of troubles because you don't really have a counter besides your own and Roar Heatran (which is just extending your loss), and so can BU Conkeldurr because Reuniclus can't hit it all that hard with Psyshock after a Bulk Up, and Tentacruel can't really hit it hard. Toxic Spikes will also boost his Attack, letting him wreck even more. I wouldn't call rain really troublesome, as Abomasnow and Kyurem can easily wreck rain teams with just two members, and rain teams hate Toxic Spikes from Tentacruel, and Reuniclus and Conkeldurr are also decent against rain if you can set up. Also something BKC forgot is Dragon Dance Gyarados (I know I said rain offense is managaeble, but Gyarados is different). It sets up on Conkeldurr, Heatran or Tentacruel (when willing to risk Scald Burn). After that, your three Water resists are all wrecked by boosted +1 Stone Edges, as Kyurem and Abomasnow are both OHKOed by Stone Edge, while Tentacruel can't do anything major while getting 2HKOed (or OHKOed by Earthquake). Conkeldurr and Heatran are also KOed by +1 Waterfall while Reuniclus is 2HKOed and can't 2HKO with Psyshock (does 45% max).
Okay, so I think the Mew BKC suggested works well, but you could also try a Sableye. That is, because it has priority on all his moves, letting him burn Gyarados and other sweepers before they attack. Sableye can Will-O-Wisp Gyarados before it gets a chance to +1 Waterfall, and can Recover (again, with priority) to outstall Gyarados thanks to burn and Life Orb. He also has Ghost-typing, letting him block Rapid Spin from Tentacruel, which can be annoying seeing how important Toxic Spikes are for Kyurem to break through the pink blobs. He also is immune to both of Reuniclus common moves, so it can outstall Reuniclus without having to Recover, and he also isn't weak to Shadow Ball, meaning Trick Room Reuniclus. It's also less risky for Sableye to directly switch in on Conkeldurr because he isn't weak to Payback, though Mew can OHKO Conkeldurr with Psychic and it's not like he takes a ton from Payback, so Mew is probably better against Conkeldurr (though they both come out on top against Conkeldurr quite easily). You probably know the set already, but here it is just in case:
Sableye @ Leftovers | Prankster
Bold | 252 HP / 252 Def / 4 SpD
Will-O-Wisp / Recover / Taunt / Night Shade
Also, you could even try a Calm Mind / Hidden Power Dark / Will-O-Wisp / Recover set. It might seem really gimmicky, but I have used it a little and it's actually really useful. I would probably stick with Taunt + Night Shade though because it takes on stall teams better.