I never used Cloyster, but (in addition to the tips from atsync and Chinese Dood) I would recommend something that can take Electric attacks and doesn't care about paralysis, because from my experience, even Gengar, who is not weak to Electric, doesn't like to stay in moves like Thunder oder Thunderbolt as the paralysis chance combined with damage will make it almost completely useless. And as Cloyster attracts Electric moves even more, such a pokemon would probably be quite helpful.
As for myself, I played some more in Pt Factory, and though I didn't get the coloured print, I managed to get a record good enought to make it in the list:
Pt Battle Factory Level 50 Singles
Streak: 45 (28 trades)
I don't remember much about the first 28 battles, but in battles 29-35 I had those pokemon (I never traded in this set of seven):
621 | Garchomp | Modest | White Herb | Draco Meteor | Earth Power | Flamethrower | Surf | SpA/Spe
756 | Metagross | Adamant | Occa Berry | Hammer Arm | ThunderPunch | Ice Punch | Aerial Ace | Atk/Def
664 | Froslass | Modest | Expert Belt | Ice Beam | Thunderbolt | Signal Beam | Destiny Bond | SpA/Spe
Most of the time, Garchomp and Metagross did the job, Froslass wasn't that good (not bad either, though), because it lacked the bulk the other two had, but it was the best backup I had.
721 | Exeggutor | Brave | Focus Sash | Wood Hammer | Zen Headbutt | Trick Room | Explosion | HP/Atk
(in battle 36 and 37, i had a Slowking, before I obtained this one via trade)
707 | Machamp | Adamant | Quick Claw | DynamicPunch | Earthquake | Stone Edge | Ice Punch | HP/Atk
(with No Guard of course)
849 | Gliscor | Adamant | Expert Belt | Earthquake | Aerial Ace | X-Scissor | Stone Edge | Atk/Def
TR Exeggutor and this Machamp set work together perfectly, most of the times Exeggutor sets up Trick Room, explodes and then Machamp kills the rest of the opposing team. All of the three have good bulk and massive attacking power. Gliscor was okay as a safety net, but in the five battles with Exeggutor it was just used once when I directly switched it into Lead Heracross.
580 | Porygon2 | Quiet | Iron Ball | Tri Attack | Thunderbolt | Recover | Trick Room | HP/SpA
805 | Slowbro | Quiet | Leftovers | Psychic | Surf | Ice Beam | Trick Room | HP/SpA
846 | Honchkrow | Adamant | Scope Lens | Night Slash | Drill Peck | Sucker Punch | Steel Wing | HP/Atk
Trick Room again, but it works great in the factory and what else should I choose with those three in the start choice?
Porygon2 can bring the TR against almost everything because of its massive bulk, it even took one Choice Band Staraptor's Close Combat (after a traced Intimidate) and set up Trick Room to strike back with Thunderbolt.
Slowbro also has much bulk (and Leftovers) and can effectively stop the Fighting types P2 has problems with. Also, Slowbro has a good offensive synergy with P2, because it can deal with Rock and Steel types very well due to its resistance against steel attacks and STAB Surf, and its Ice Beam combines well with P2's Thunderbolt, and of course there's Psychic, that makes beating fighting types much easier. Honchkrow rounds the team off, and is able to act effectively in as well as outside of TR due to STAB Sucker Punch.
I lost in battle #46 to:
767 | Swampert | Adamant | Rindo Berry | Earthquake | Aqua Tail | Avalanche | Mirror Coat | HP/Atk
785 | Vespiquen | Adamant | Shell Bell | Attack Order | Defend Order | Heal Order | Confuse Ray | HP/Atk
882 | Kingdra | Modest | Wide Lens | Hydro Pump | Draco Meteor | Blizzard | Signal Beam | HP/SpA
Swampert is a difficult foe for Porygon2 and my whole team. I can't stall it out with Recover because Earthquake has good chances to 2HKO and it would be just a matter of time until a crit happened (okay, it has only 10 PP, but still, that would be a risky strategy).
So I brought out TR first and he used Avalanche which didn't do really much. Then I used Tri Attack, hoping he wouldn't go for Mirror Coat. He didn't, but used EQ, which did roughly 50%. Then I used Tri Attack again, and he killed P2 with Avalanche.
Slowbro comes in and KOes Swampert with Surf.
The opponent sends in Vespiquen, I use the last turn of Trick Room to get an Ice Beam off, which did slightly over 50% damage, he uses Confuse Ray (Slowbro had Oblivious). I switch to Honchkrow, which takes around 50% from Attack Order. Then Drill Peck KOes him.
The opponent sends in Kingdra, and as I'm not sure what attack he would use I stayed in (if I was sure that he'll use Blizzard, I would have switched to Slowbro and then back to Honchkrow to avoid that Slowbro has to take a full power Draco meteor; if I switch Slowbro into Draco Meteor, though, he can probably just 2HKO it and then Honchkrow must survive at least two hits (or get a crit), which it can't even against a -4 Kingdra, as long as it only has half of its health) and did as much damage as possible with Sucker Punch (~45%), before he killed Honchkrow with Blizzard.
Than I send in Slowbro, Draco Meteor brings it down to ~28%, Slowbro brings TR, Leftovers heal it to ~34%. A -2 Draco Meteor does 31.7% - 37.6%, so I have a chance to survive, outspeed and 2HKO Kingdra. Psychic does 48,4% max with Psychic, so it will never OHKO without a crit, Ice Beam does 28,6% min, which is (if my guess of ~55% remaining HP was correct) enough for a 2HKO, but also brought a 8% chance of a freeze for at least one turn with it. So i went with Ice Beam, and it indeed froze him... but he immediately thawed out and killed Slowbro with a Draco Meteor crit.
I forgot to save the video, so here's a picture proof: