I'll rank a few mons
Sudowoodo
Availability: Pyrite Cave: Miror B.
Stats: It has great Attack/defense and average HP, but awful speed, special attack and below average sp. defense
Typing: Rock is good offensively but pretty bad defensivly. Since it's very slow it will often go 4th in the turn which means it can typically get hit twice. It's weaknesses are plentiful as attacking moves in Colosseum.
Movepool: Movepool is okay. It comes with rock slide and low kick, which have its uses throughout the game, but block is fairly useless and its too slow to take advantage of flail. The only other moves it learns are faint attack (which is trash due to its trash sp. attack) slam (lmao) and Double-Edge (which is a good move, but comes at 57, just a little bit too late of Evice unless you do a lot of offset grinding)
Major Battles: Dakim 1&2's normal pokemon will butcher Sudowoodo with EQ spam, while mostly resisting its STAB as well especially since it might not be purified (round 1), but it does decent against Entei, resisting fire blast. It can't do a lot against Venus 1 or 2 and will most likely get hit by her annoying status moves, as well as getting hit super effectively against most of her moves but sometimes outspeeds her Steelix and sometimes can live its Earthquake. Ein 1&2 is a Rain Dancing Toxic staller but can have some success due to half of Ein's team in both battles being flying type. Miror B 2 doesn't appericate you stealing his Shadow Sudo, so don't even bother. Gonzap relies on bulky offense (mostly) and can hit sudowoodo effectively with all of his pokemon. Nascor is mostly Special based with some confusion moves but Sudowoodo doesn't apperciate getting hit on the special side. Some of those moves are SE as well. Evice typically hits too hard for poor Sudo due to Evice relying on self stat boosting, but has a good matchup against Salamence and his Shadow Tyranitar.
Additional Comments: Sudowoodo wont be doing much for any of the major battles as nearly all of them exploit its crippling weaknesses. It also has 10,000 points in the shadow gauge so it takes quite a while to purify. It does help a bit with capturing shadow pokemon as shadow rush is a physical move and your options for physically defensive pokemon are fairly scarce, and it does come pretty early in the game.
Overall I'd put it in Low or D tier due to its poor showing in all major battles as well as taking a long time to purify, but it's not completely useless like Aipom.
Foretress
Availability: Shadow PKMN Lab: Cipher Peon Vana
Stats: Fantastic defense state, decent enough attack stat, below average special attack and special defense.
Typing: Bug/Steel is an interesting typing. Immue to poison, only weak to fire, and resists 10 typings. But Foretress can't use its typing offensively without Hidden Power for reasons stated below.
Movepool:It's level up movepool is terrible. Since Colosseum adopts RS level up movepool it is incapable of learning Zap Cannon. All it comes with are normal moves and only learns spikes and double edge while leveling up in Colo. You wont be using Rapid Spin ever, but at least it comes with protect and gains explosion when purified. This means its extremely reliant on TMs but at least its TM movepool is pretty decent. It can learn things like toxic, Dual Screens, and even EQ. It also learns giga drain, which has its uses against SE targets. Explosion, which for the explosion turn only pre Gen 5, also lowered the opposing Pokemons (and your partners) defense by half, which means unless its severly lowered level, it will KO anything that doesn't resist it or is boosted by reflect/defense raising moves. You'll most likely be relying on Hyper Beam on offense unless you get a really good STAB hidden power.
Major Battles: Dual Screens and Exploding have its use in all of its attending battles. Just be careful against Dakim 2 because he also utilizes Sunny Day strats.
Additional Comments: 6,000 points in the shadow gauge means it takes some time to purify, but not too long and but it does come pretty late.
Overall I would put it in Low or on the very bottom edge of C tier. It does require babying if you get a cruddy hidden power (which is very likely) but dual screens and 500 BP explosions are nothing to laugh at. Getting rid of two problematic Pokemon while Protect TMs are cheap, plentiful and everyone can learn it means you aren't sacrificing much.
Meditite/Medicham
Availability: Pyrite Cave: Rider Twan
Stats: Meditite/Medicham's defensive stats are pretty average, with its hp and offenses being below average and its speed is above average but its main draw is its ability Pure Power. Medicham might have a base 60 Attack stat, but it actually has a whopping equivalent 174 base attack if fully invested or 123 if RNG happens to hate you. a pseudo-123 base attack is still great and hits plenty hard. It's speed stat is 80, which isn't all too fast, but will still outspeed the majority of the Pokemon in the game.
Typing: Fighting/Psychic is good offensivly and average defensively. it's resistances are fairly common in Colo and its weaknesses only really show up around some bosses.
Movepool: High Jump Kick, High Jump Kick, HIGH JUMP KICK!! Its really all Medicham needs, it even comes with it when its a Meditite. Even Pokemon that resist fighting moves get chunked fairly hard. But it does get Shadow Ball via tm for unresisted coverage (though you'll have to go a bit out of the way for it) , calm mind, if you REALLY wanna use it for special attacking, and can learn dual screens for some support. Teaching it Focus Punch might be overkill and has risk involved, but if you can land it, it will decimate anything that it hits. Psych Up is actually pretty handy in Colo as the Cipher Leaders like to boost their stats, but other than that it's not all that useable since your own Pokemon dont have access to many boosting moves themselves, unless you're a dirty flipping steroid abusing X item chugging commie
Major Battles:Miror B is not familar with the dancing style of the Cancan, so demonstrate it to him. He doesn't even hit all that hard due to his moves anyway. Medicham doesn't have much to fear from Dakim 1. Just kick him in the nads for revenge for being one of the very few characters in the entire series to inflict Human on Human violence on your friend. Venus 1 gets cnt punted, just watch out for Banette though. Roundhouse kick Ein in the face when his flyers are down, though if he gets a rain dance up, his swift swimmers will typically outspeed and OHKO Medicham unless you have a Light Screen up. Dakim 2 is mostly the same as Dakim 1 but he does have a Claydol this time around which can be pretty bothersome. Venus 2 removes her Banette so you don't have to worry about potential immunities and her new pokemon are typically weaker on the physically defensive side. You'll outspeed most of them anyway. Ein 2, like Ein 1 is troublesome as he has some fast pokemons that will typically outspeed and OHKO, and has fliers as usual but he also has a Rhydon. Rhydon is simply free to Medicham, but Medicham won't take offense if you bench it for this battle. Medicham does pretty well against Gonzap resisting his fighting moves and hitting his dark types super effectively too. It can do well against Nascor but try to keep it away from his Gardevoir, Xatu, and Dusclops (unless you have Shadow Ball). Evice's Slaking is free to a kicking if you have protect (and if Slowking isn't out at the same time), can psych up Scizor's/Machamp's SD/Bulk Up or even Silver Wind if you're lucky enough, but try to keep it away from Salamence. Shadow Tyranitar gets bodied, typically.
Additional Comments: Overall I would put Meditite in High tier or A. It comes barreling out of the gate with HJK and hits hard from the beginning. Even though it evolves at level 37, its only 4 levels away since its caught at level 33, which will be 2-3 due to the purification process. It also has 5,000 points of shadow gauge, which is higher than the beginning shadow mons, but not by much so it should take maybe a half hour at the most to deplete its gauge. Medicham just hits ludicrously hard to pass up and fairs well against most major battles, though it is a bit luck reliant if you rely on HJK due to its 90% accuracy. You won't miss a lot, but when you do, it literally will hurt and can mean the difference between you or the enemy being KO'd. Even if you don't teach it other typings of moves, its pretty similar to Espeon as it can just use its STAB to maim most encounters in the game. Depending on how others feel, I might even consider it for Top or S. The only thing that's really stopping me from doing so is that unlike Espeon, the things that can potentially OKHO it for the most part can outspeed it.