B2W2 Cheren can randomly be such a horseshit fight in Hardcore Nuzlockes, especially on Challenge Mode. This is why it is both the best and worst fight in the series imo.
You have exactly four areas to get Pokemon before the Gym: your starter, Route 19, Route 20, and Floccesy Ranch. You are rolling up the first Gym with at most four Pokemon. Which isn't awful, Cheren only has 2/3 Pokemon after all. But the encounter tables for these areas can completely screw you:
Route 19: Patrat / Purrloin - Patrat is slowish and has generally bad stats and Purrloin has 0 good attacking options. Both of these Pokemon are generally terrible and unless you get blessed with amazing IVs and natures are not exceptional against Cheren in any way.
Route 20: Patrat / Purrloin / Sunkern / Pidove / Sewaddle - Sunkern has ultra gabarge stats, but at least it can Grasswhistle and Growth if you want to risk it for biscuit. Pidove is okay with potential Super Luck Quick Attack, and it is actually reasonably bulky for the first fight, but in the end it is still just a Pidove. Sewaddle is the best one here: 70 base Defense and a meaty STAB Bug Bite this early means it can actually put in work for this fight.
Floccesy Ranch: Lillipup / Azurill / Patrat / Mareep / Psyduck / Pidove / Riolu - If you're Lillipup has good Speed its a decent mon for the fight since it has a strong Tackle/Frustration and can flinch with Bite. Azurill is a 50/50 with its ability, but even then gets no STAB outside of Frustration. It also is slow AF. Mareep can sometimes Static which is nice and ThunderShock does decent damage. Psyduck has a decent Water Gun but not much else. And while Riolu seems awesome, and to be fair it is with Force Palm, its only a 5% encounter.
So generally speaking you have your starter, a weak mon, probably a second weak mon from Route 20, and hopefully an okayish mon from the ranch. I have had runs where I got my starter, Purrloin, Sunkern, and Patrat as my team and it is absolute AGONY. PURE AGONY.
But Cheren himself also has the advantage of being able to use Work Up and Bite. If his AI decides to use moves in a somewhat decent order, he can start throwing out +1 50 BP STAB Tackles at your poor early-game squad. If he outspeeds you, he can also randomly click Bite and flinch you into oblivion. And in Challenge Mode, Lillipup gets an Oran Berry and he adds a Super Luck Pidove to his team. At this stage in the game, you don't have the EVs to compensate for poor IVs or natures on your Pokemon. And the starters in Unova aren't exactly exceptional; with the 13 level cap, their best STAB moves are Vine Whip, Ember, and Water Gun for the fight. And if you picked Snivy, you're damage is lacking unless you can manage to squeeze in some Growths. Tepig and Oshawott can be slower if you get bad natures or IVs, too. Oh, and Cheren can use a Potion.
B2W2 is the only Nuzlocke I will reset in if I get a bad starter because the chances of me having a somewhat decent Cheren fight are just so much worse that it isn't worth the time to even run it imo. The worst part of the fight is its super inconsistent; sometimes Cheren is just an idiot and spams Work Up and never bothers to attack. But this degree of BS is why its also the best; coming out of Cheren relatively unscathed is such a high that it sets the tone for the rest of the run.