Hey there !
I'm not the one who made this guide, HustleBank did, but I'll try my best to explain shortly how the strategy works. I'm not sure how experienced with pokemon you are, so I'll go in more in-depth with some details regarding this team. It will be useful as well if someone else needs it. Using this will require a bit of practice against certain bosses if you're a beginner in this area.
The first thing to take in consideration here is the multiple pokemon roles and advantages :
Defense :
Klefki: Will be your primary line of defense. Klefki has access to the ability Prankster. This gives priority to status moves, which means that you will go first in a boss battle. Klefki will be used mostly to put screens. The screens will double your pokemon defense or special defense, allowing you to tank more hits. (Example of screens: light screen, reflect)
Whimsicott: Whimsicott will be your principal de-buffer for pokemon that uses physical attacks. Charm will reduce the opponent's pokemon attack. It also has prankster, like klefki. Memento/flash/knock off will be useful too.
OR/AND
Ampharos: Will do the same as Whimsicott, but with spatk. Eerie impulse is the move to lower spatk.
Gliscor: Will increase his attack and speed to use baton pass and give those buffs to conkeldurr. Using the move stealth rock also, which will make focus sash useless and assure that conkeldurr can one-hit everything. The combination of poison heal and toxic orb allows you to not get burned or frozen while healing you.
Chansey: Casual stall pokemon that allows you to toxic and heal if something goes wrong. (Which rarely ever happens)
Offense :
3 different options here, depending on what team you are facing.
Conkeldurr: Will be the one who kills all the boss pokemon, 90% of the time.
Cloyster: Used on magic guard alakazam that have sashes, because this ability blocks stealth rock damage. Cloyster has multiple skills that hit multiple time and this will ignore the focus sash effect. (Icicle spear for example)
Mold breaker pokemon (Haxorus, Excadrill, etc): Used against unaware pokemon, since these pokemon ignore stats changes (Like de-buffs and buffs). Mold breaker ignores the opponent's ability, unaware in that case, so your attack buffs won't be ignored.
Other :
Gengar: Uses destiny bond if a pokemon could cause problems while de-buffing.
The nature listed for each pokemon is the most optimal in terms to get more defense or attack. This will assure that you beat the bosses.
The short summary of how a battle goes :
Let's take a boss for example; Erika.
1st turn: Klefki➔ Reflect (Your pokemon will tanks physical hits better). Switch to Whimsicott.
Turn 2, 3 and 4: Use charm 2 times with Whimsicott, then memento. (Will reduce the attack to the lowest point possible)
Turn 5 to 11: Choose Gliscor. Use rock polish 3 times, Sword dance 3 times. (Buff your attack and speed to the highest level possible)
Turn 12: Stealth rock (Against pokemon that hold focus sashes.)
Turn 13: Baton pass your buffed stats to Conkeldurr.
Turn 14 to the end: Kill all the boss pokemon with the moves provided.
I hope you got all the answers you needed. Feel free to dm me if you are having issues or have more questions regarding this strategy. ~