Ok, so beating this thing defensively is almost impossible, which is why I tried to do so:
Gurdurr @ Eviolite
Ability: Iron Fist
EVs: 252 HP / 192 Atk / 64 Def
Adamant Nature
- Drain Punch
- Mach Punch
- Ice Punch
- Knock Off
+2 252+ Atk Incineroar Flare Blitz vs. 252 HP / 64 Def Eviolite Gurdurr: 280-330 (74.8 - 88.2%) -- guaranteed 2HKO
192+ Atk Iron Fist Gurdurr Drain Punch vs. 0 HP / 0 Def Incineroar: 290-344 (87.6 - 103.9%) -- 25% chance to OHKO
192+ Atk Iron Fist Gurdurr Mach Punch vs. 0 HP / 0 Def Incineroar: 156-186 (47.1 - 56.1%) -- 76.6% chance to 2HKO
As you can see, if you manage to keep it above 88% you always tank a +2 Flare blitz (I recommend playing impeccably, or using wish support), while you trivially win with Drain Punch, and even do more thank half with Mach Punch. Again, it ain't a perfect counter, but it definitely limits Incin's ability to break a team, and also contributes nicely vs Heliolisk.