Since I started Bossing with Batton Pass (after learning with Dragon Dance Gyarados at lowest difficulty) I am late to this team.
I am currently trying to have an alternative Sweeper for some bosses by switching Mega Slowbro to a Magic Guard Clefable.
It needs:
Bold/Relaxed nature. 178 Def, 80 SPDEF, 252 HP (imo better)
Calm/Sassy nature. 252 DEF, 252 HP.
Since it does not have Iron Defence I changed up the movepool:
Cosmic Power --> Raises Def + SpeDef
Softboiled - Healing Move, TM available in Fuchsania, elso get Moonlight (Lvl up/Move Relearner)
Chare Beam --> weak special electric attack, that has 70% chance to raise SpeAck available after defeating Surge. It is advantagous to raise PPs of this move
Main Dammaging Move
Magic Guard allows the Pokomen to avoid indirekt dammage so it can start burned against Bosses with freeze or paralyse or it can ignore Toxic/Poison/Burn.
The disadvantage is, that you loose imunity to crits, so rng can screw you over there.
The strategie is, that Clefable comes in after debuffing and hazards, sets up 6 Cosmic Power, Uses Charge Beam to rais SpAtk and then switches to main dammaging move. All while healing if nessesarry. You may also switch to the other move earlier and go back to Charge Beam for some Poke later (or even come back with maxed SpeAtk for effectivenes)
As for which one to chooce there are the following options:
Stored Power: Probably the best one, it is the special counterpart to Power Trip, from Bhimoso's Batton Pass guide. To paraphrase his words from there: Stored Power's base power is based on the amount of stat boosts you have. It powers up more the more stat boosts you have. Stored Power's maximum base power for this setup is 380. Yes, you read that right: 380. Clefable gets up to +6 stats on Def, SpeDef and SpeAtk, clicks Power Trip and faints everything. Well everything not imune to Psychic, the biggest weaknes for Stored Power. Also, even if you only have +6 on Def and Spedef you attack with 260 Base Power (and only Base SpeAtck), so it's not a clean sweep but it can get close. Otherwise this option is simular to Psychic on Slowbro.
Moonblast: The STAB-Alternative. It has better coverage so the floor is lower. The cealing too thought.
other Options lose out but they exist:
Icebeam: Probably the best, with Charge Beam it is "BoltBeam at home" Best coverage, but slowest, simular to the same move on Slowbro
Flametrower: Again, simular to the same move on Slowbro, Has less coverage but faster for some bosses.
Shadow Ball/Solar Beam/Signal Beam/Focus Blast/Tri Attack/Hyper Voice --> Scraping the Barrel, these are 95% the inferior option. Maybee great for one specific boss, would need to look it up, but normaly you want broader coverage.
Bosses that should be easier with this Pokemon(decending in Rank from the list above):
Lt Surge -- Lorelei -- Pewdie & Diepy-- Thor -- Terminator -- Klohver -- Battlebot -- Naero
(still beware the flinches for some of them)
Aditional testing needet to confirm this.