Shellos is a fall back option really. Personally, it all comes down to the team building aspect, and what to look out for.
Actually, it all comes down to personal preferences. We cant just assume one pokemon is awesome and in the A grade just because it can be a sweeper, or help others. Shellos is underrated to the extent tat i see almost noone ever uses it. Even so, it is used wrongly.
As you mentioned, Shellos has only 2 weakness, and i already stated these are it's disadvantages. But if you were to look at teams now, you will see looks at Mienfoo, Timburr, Pawniard etc all the S to A rank pokemon running wild. And in my experience in battle, Shellos has never failed to disappoint me to be able to keep track on these pokemon. Mienfoo can't even hurt Shellos, as Drain Punch would jsut deal 33.3% to my Shellos, resulting in me getting a free scald attack, or recover, forcing it to switch. Now, prediction comes into play. What can I do? If electric is coming in, Earth Power greets them, while clear smog meets my other threats.
The idea of Shellos:
1. Hit hard to deal damage over time.
As you mentioned, this game is a volt/u-turn populated. You can either play safe and switch appropriate, or you can be smart and damage it hard enough to cut down this trend. My example: Mienfoo vs Shellos. It uses U turn, and guessing Shellos is trying to get the lucky burn. Chinchou happily take the water attack, but what happens? Earth power greets them, serverly limiting their volt switch number, and shellos happily switch out with a better matchup to a nice advantage.
Facing a Pawniard what can you do? Iron Head is definitely not going to scratch Shellos, knock off can't take out evolite. Your idea of getting 2HKO is strange, as I said one burn is enough to disable it, leading to recover. Also you can spam recover to keep it healthy till it can survive 2 hits to try for another burn. If opponent do switch to electric/matchup, at least Shellos is healthy enough when switch out, allowing my team to have a better matchup.
The ability Sticky Hold is in fact in my opinion, one of the best ability in the LC game, as LC focuses a lot on knock off. Almost every pokemon will knock off one's item, especially evilote to ensure more damage will be done. Shellos blocks that well, and with recover, it doesn't really matter about it's typings. as i said earlier, most fighting mon dont even have a electric or grass move, barring thunder punch. A 1v1 with them would be in favor of Shellos if you used it well.
"Any higher would be absurd and an insult to the extremely good mons in B+ or higher."
I will personally analyse some of the pokemon in the higher rank, to see if that's really an insult, or people won't admit it's good as noone uses it.
S Rank : Mienfoo, pawniard
As stated just now, there is virtually no way these 2 mon can 2HKO it without boosting it's stats. Also, Shellos run 212+ def, not 132+. Drain Punch Mienfoo ( 37% max) Knock Off pawniard (40% max). Definitely enough for Shellos to use recover to get it back to healthy mode and get the burn to decide the tie.
thus Shellos pretty much walls these 2 mons.
S- rank : Abra, Timburr
Abra is a bit problemetic. It can probably deal around 40-50% with psychic, or 80-90% with energy ball. Unless your team is well build to be able to cover Shellos, you can go ahead and destroy the sash and finish abra off with a priority move or so on.
Timburr with guts can be unhealthy to Shellos, drain punch around 38-50%, but nevertheless, there's move like earth power u can use to get a lucky drop in sdef. But, it's bulk up is completely neutralised by clear smog, giving it no chance to setup. When used earlier in the battle, if Timurr switches, you have to see your opponent team and predict which mon is coming in to get the adbvantage in this turn/volt battle.
Shellos finds it evenly match with Timburr, and Abra might be a problem to Shellos iff Abra had energy ball, but if your team can affor a shellos sacrifise and a mon to finish it off, this gambel might not be that bad.
A+ rank: Archen, Fletching Magmemite, ponyta
it is a bit disheartening when u keep mentioning shellos defence is nothing. but i used shellos in so many battle tto know defence is something.
Against Archen, it's strongest move Stone Edge, can probably do around 40-50%. yes I really went to calculate it. Shellos here, can go for a scald, knowing 2 stone edge wont kill it. Also Scald will defeat Archen most of the time, or even leave it with a burn, and defeatist. So Archen is really bad against Shellos.
Fletching, surprisingly, can only deal 33-37% with boosted acrobactic. Clear Smog would be useful here if it decides to go for sword dance. Once shellos recover to the point, it can withstand 2 hits, scald comes in, dealing more than 50%. Process repeats itself, and it's made simpler if there was a burn.
magmemite: This is really risky, as you need to hope that they do come in from a uturn or volt switch to wreck it with earth power, after Stealth. However, if it comes in on a free switch, just run. Get a spdef bulky pokemon like munchlax to tank it. Avoid at all cost
Ponyta: This get interesting as Shellos Scald can hurt it badly (60-70%). Wild Charge deals around 60% to Shellos, since Shellos is def invested. Morning Sun Ponyta isnt befitting it , as Scald does more damage then the recovery. Also, Ponyta has to risk koing itelf from wild charge should it really go suicidal and kill itself, bring shellos down itself.
Shellos can almost match up with the offensive and defensive capabilities of these mons. You just have to use it to believe it