Assuming a +40 base attack increase:
252+ Atk Primal Groudon Heat Crash (100BP) vs. 4 HP / 0 Def Xerneas in Sun: 375-442 (95.1 - 112.1%) -- guaranteed OHKO after Stealth Rock
252+ Atk Primal Groudon Flare Blitz vs. 4 HP / 0 Def Xerneas in Sun: 450-531 (114.2 - 134.7%) -- guaranteed OHKO
252+ Atk Primal Groudon Heat Crash (60BP) vs. 4 HP / 0 Def Kyogre in Sun: 118-139 (34.5 - 40.6%) -- guaranteed 3HKO after Stealth Rock
252+ Atk Primal Groudon Flare Blitz vs. 4 HP / 0 Def Kyogre in Sun: 235-277 (68.7 - 80.9%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Primal Groudon Heat Crash (80BP) vs. 252 HP / 0 Def Arceus in Sun: 247-292 (55.6 - 65.7%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Primal Groudon Flare Blitz vs. 252 HP / 0 Def Arceus in Sun: 370-436 (83.3 - 98.1%) -- 68.8% chance to OHKO after Stealth Rock
252+ Atk Primal Groudon Heat Crash (60BP) vs. 4 HP / 0 Def Palkia in Sun: 54-64 (16.7 - 19.8%) -- possible 5HKO after Stealth Rock
252+ Atk Primal Groudon Flare Blitz vs. 4 HP / 0 Def Palkia in Sun: 108-127 (33.5 - 39.4%) -- guaranteed 3HKO after Stealth Rock
252+ Atk Primal Groudon Heat Crash (100BP) vs. 0 HP / 0 Def Yveltal in Sun: 375-442 (95.4 - 112.4%) -- guaranteed OHKO after Stealth Rock
252+ Atk Primal Groudon Flare Blitz vs. 0 HP / 0 Def Yveltal in Sun: 450-531 (114.5 - 135.1%) -- guaranteed OHKO
And these are only the S and A+ threats in ubers where Heat Crash and Flare Blitz aren't at the same base power. Out of all of them, only Xerneas and Yveltal are guaranteed OHKOed by both.