I really like this idea. Something that should be noted about Bulletproof Chespin is that it actually is immune to both of Gastly's STAB moves. It also walls standard Bellsprout as well as Tentacool (though Knock Off will be annoying). This could contribute to Gastly running Sludge Wave over Sludge Bomb if it becomes common enough, but Bellsprout does not get Sludge Wave, so it's forever walled.
Effect Spore a 30% chance to status, not 10%. Please don't divide it up as 10% for each to make it seem less important; it's a 30% chance to afflict one of those three on contact. Nobody says Tri Attack has a 6.66666666667% chance to either burn, freeze, or paralyze... they say it has a 20% chance to inflict one of those three. :)
30% is not so dependent on luck that it's not useful. It's the sole reason players use Flame Body over Flash Fire and run Scald on almost everything instead of the more powerful Surf. If you realize how often Scald burns happen or how often Focus Blast misses, Effect Spore has the same chance of activating and doesn't even require Shroomish using a turn; only that a foe touches it, which can be expected to occur reasonably often if it's used as a physical wall. You know that feeling that rushes through your mind when the opponent uses Scald on your physical sweeper or Body Slam on your fast hard-hitting attacker and you pray to yourself (PLEASE DON'T GET THE STATUS HAX)? Lol same thing here.
196+ Atk Fletchling Acrobatics (110 BP) vs. 196 HP / 196+ Def Eviolite Shroomish: 14-20 (56 - 80%) -- guaranteed 2HKO after Stealth Rock
196+ Atk Fletchling Acrobatics (110 BP) vs. 68 HP / 76 Def Eviolite Chespin: 18-24 (78.2 - 104.3%) -- 6.3% chance to OHKO after Stealth Rock
Eviolite Chespin's bulk with your set is 23/21/21, and Eviolite Shroomish's bulk with my set is 25/24/21, making Shroomish notably more bulky. Your Chespin is designed to also be able to attack though, which is pretty cool, but its main STAB attack does not simultaneously heal it like Shroomish's Giga Drain. The lengthy list of weaknesses that comes with being pure Grass-type means that you'd want to get as much health back as possible. Also, Chespin is disadvantaged by burn status while Shroomish isn't. Walling Gastly, Bellsprout, and Tentacool is very cool, though. Both have their advantages and disadvantages, but I wouldn't say that Chespin is outright better.
Also, non-STAB U-turns really don't hurt Shroomish very much and enemies also risk that 30% Effect Spore chance by using it. The only viable user of STAB U-turn in Little Cup is Larvesta, which Shroomish and Chespin have no business trying to stay in against anyway.
I'm happy that this thread has people thinking, because I think LC players are realizing here that there are more usable Drilbur counters than they thought. A very interesting one that I messed with later tonight is this:
Seedot @ Eviolite / itemless
Ability: Pickpocket
Level: 5
EVs: 196 HP / 156 Def / 36 SpA / 116 SpD
Bold Nature
- Leech Seed
- Synthesis
- Giga Drain
- Defog
Pickpocket Seedot is an interesting answer to Drilbur that also punishes contact moves from physical attackers in a different way. This thing not only tanks Drilbur's Earthquake, but it can also remove Drilbur's Stealth Rock with Defog, something that Foongus, Shroomish, and Chespin cannot do (though I strongly feel that Chespin should have gotten Rapid Spin). The really interesting thing about this set, though, is the ability. Pickpocket 100% guarantees item theft if Seedot is without an item and an enemy with an item makes contact. This would not only deter players from Knocking Off its Eviolite, but also make common physical attackers that rely on their items think twice about touching it when it's itemless. It can also just go itemless from the start and steal an item from the first thing that touches it, which is pretty rude. Reliable healing with Leech Seed and Synthesis makes it hard to bring down from the physical side without a hard super effective hit and it can serve as a pretty cool Knock Off shield for a team, as it can consistently steal more and more items from the opponent's Pokemon simply by being touched. It's kind of mean to be honest; in no other tier is a Pokemon's item SO important to it.
I'm not saying Seedot is whoop-de-doo top tier material, but it is pretty cool to use and will definitely catch some players off-guard. :)