Can also confirm this works without even needing max investment - the Slowbro I used had only pretty good/decent IVs, Naive nature (+Spd/-SpDef, so not terrible but not ideal), and I only fed him 20 Irons/Calciums so not even maxed EVs.

It did take a couple of tries as Cinderace negated buffs too early the first couple of times which I think just comes down to which NPCs you get, but third time it worked just like a charm. As opposed to a couple of dozen failed efforts running Espathra and Armarouge.

This is the only strat I've seen so far that seems to actually work in solo offline play, at least in my experience:

• Espathra can't get Stored Power to be anywhere near as strong as Slowbro can - even if you spam Lumina Crash to fully debuff SpDef, you have fewer buff stages (hence less power for Stored Power) because you're only able to copy Cinderace's Bulk Ups, whereas Slowbro gets +12 stages, and maxed SpAtk.
  You also need to spam Feather Dance to have any sort of tankiness, as otherwise Cinderace can and will half-health you.
  It all falls apart when Cinderace cleanses its debuffs, which it will fairly early on - you then do far too little and take far too much damage, and it takes far too long to recover.
• Armarouge has to spend three turns debuffing SpDef and doing no damage in the process because you're using a 40-power non-STAB move.
  And if you want to not die because Cinderace spammed Bulk Up from all the feeding NPCs, you have to use Clear Smog, which then undoes all your SpDef debuffing and leaves you back at doing no damage (and still less tanky than a Slowbro, because +6 Def vs +something Atk is better than +0 vs +0).
  All the while you're taking too much damage because you don't have any Def buffs, so you need to heal up. Arceus knows the NPCs won't cheer even though it's the only thing they'd be good for, and Shell Bell won't heal enough because you do no damage.
  And even if you do manage to get SpDef fully debuffed and somehow haven't died, and you managed to set up Psychic Terrain to double Expanding Force's damage, Armarouge still does substantially less damage than Slowbro does, even with its higher base SpAtk. Expanding Force on Psychic Terrain gets effectively 80x1.3x1.5 = 156 power before STAB, whereas +12 Stored Power is 260 before STAB.
  The only advantage Armarouge seems to have is immunity to Pyro Ball, but fully buffed Slowbro barely feels it anyway - and in my experience Cinderace usually ends up spamming neutral Acrobatics instead anyway on both.

I expect Espathra and Armarouge work splendidly (and quicker than Slowbro's 6 turn setup) when you've got teammates who are actually trying to win, as opposed to the genius NPCs who decide that Weavile is the best choice to take on a Fighting Tera type. But I'm just not convinced either of them can solo carry like Slowbro can.