252 Atk Life Orb Infernape Flare Blitz vs. 252 HP / 0 Def Aegislash-Shield: 307-367 (94.7 - 113.2%) -- 68.8% chance to OHKO
252 Atk Life Orb Iron Fist Infernape Fire Punch vs. 252 HP / 0 Def Aegislash-Shield: 234-276 (72.2 - 85.1%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb Iron Fist Infernape Fire Punch vs. 252 HP / 232+ Def Skarmory: 190-226 (56.8 - 67.6%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb Infernape Flare Blitz vs. 252 HP / 232+ Def Skarmory: 252-299 (75.4 - 89.5%) -- 12.5% chance to OHKO
Those calcs should tell you enough. Fire Punch does not have the power to kill things. Infernape is a brute Pokemon who should come in and rape stuff. Don't bother to try keeping it alive longer, you'll just waste time doing that.
Instead what you can do is run a mixed set with Expert Belt, Overheat, Close COmbat, Thunderpunch, and Mach Punch/U-Turn. This will prevent Infernape from taking the recoil damage, but still does some crazy damage. It will keep him alive longer as well.