The decision to go into Juan with a level 32 Lanturn and then dumping on its performance is perhaps the best illustration of why leveling pokemon by an even experience distribution is unnatural.
Choosing to not feed the Pokemon that's come late and somewhat underleveled the incredibly free KOs lurking pretty much everywhere is pretty baffling.
For Gorebyss, remember that it starts with a steep cost of having to go well off the beaten path and do the scanner sidequest, something that definitely takes longer than feeding a Chinchou some water types.
ooooooooooooooooookay
so, I retried after having battled every trainer in hoenn before juan, including those I deliberately avoided battling with mudkip and tentacruel through the whole game to avoid overleveling (which is
very generous, a normal team wouldn't be able to invest on lanturn like this), except for the people in meteor falls and the victory road for obvious reasons, because clearly it's unfair to judge a late-joining pokémon with a slow leveling rate without having grinded it for 10 levels. I was going to do a huge grind with the remaining trainers anyways for the elite four, but apparently lanturn's logs in particular have disturbed some people and want a "fairer" take on the bioluminescent fish.
the results are following:
I think you're underestimating the absurd amount of experience (and time, honestly) that goes into leveling up, unless you're pretending that training up a pokémon fifteen levels underneath our party that grows slowly is as efficient as using what we already have, or makes up for the lost time, or that it is reasonable to think that you can compare a Pokémon you have dedicated a lot of time to it exclusively to make up for its inconvenient joining time with other Pokémon that haven't needed that
The sad thing is that even with all of that, the results aren't really that good:
Juan (post-grind)
Swampert Lv54: Earthquake OHKOs everyone except for Kingdra, who faints against a combination of Mud Shot and Earthquake.
Tentacruel Lv51: Secret Power 2HKOs Luvdisc, 4HKOs Sealeo and Crawdaunt and 5HKOs Kingdra. Blizzard 3HKOs Kingdra. With the Sludge Bomb TM, Tentacruel OHKOs Luvdisc, 2HKO Sealeo and Crawdaunt, and misses the 2HKO against Kingdra by very little. With the Giga Drain TM, Tentacruel OHKOs Whiscash.
Lanturn Lv42 (w/ Magnet): Thunder OHKOs Luvdisc, Sealeo and Crawdaunt, but misses out on the 2HKO against Kingdra. Surf 3HKOs Whiscash (2HKOs in Rain), which 2HKOs with Earthquake and can complicate the match-up with Amnesia.
Gorebyss Lv48 (w/ Sitrus Berry): Psychic 2HKOs Luvdisc, Whiscash and Sealeo. Surf 2HKOs Luvdisc and Crawdaunt. Blizzard 2HKOs Kingdra, although Double Team can disrupt the match-up.
......
Now, you may argue that, in fact, it's unreasonable to expect to use a Lv54 Swampert against Juan, but do you see where I'm going with this? The amount of time/investment/effort that I needed to get Lanturn to a reasonable combat level has been the same as getting Swampert or Tentacruel to ridiculous levels that at that point you are dealing with the rest of the challenges effortlessly (presumably), and the match-ups for Swampert/Tentacruel aren't conditioned to whatever Lanturn has effectiveness to. The issue, right now, is not necessarily that Lanturn doesn't match Swampert (that's a huge bar), or Tentacruel (that's a red flag because they share tiers, but it's not a deal breaker), but rather that Gorebyss, a Pokémon that is ranked as F, is right now more appealing to use than Lanturn, a Pokémon that is ranked as B.
The fact is that Chinchou, under the experiences of this playthrough, doesn't match the description of a B Tier you have there because of the aforementioned paragraph, because Chinchou's low level, low offensive stats and slow leveling rate holds it back enough that you need to have advantage against your opponents since otherwise you won't be able to OHKO or 2HKO (see Kingdra), and the result is that it doesn't make up for its late arrival. That's why I'm asking why Chinchou is B to begin with, which you haven't responded to, btw.
Greil Mercenaries in the house!
Can't help but notice the monotype team and more importantly, the one mon show until post-Norman.
Pardon the bluntness, but are you sure you're actually running a sensible test and not doing just a fun run?
what do you understand under "a sensible test"?