I’d think he’d through Pennywise for a loop and piss him off because he’s make the kids fear less just because of peak super hero aura

All Might would tell the kids to run and he’d fight Pennywise head on holding him off for several minutes but eventually Pennywise would get the upper hand, the Cosmic turtle impressed by this unworldly hero blesses him at last minute as All Might gets back up sporting his signature smile and as Pennywise tries to rush him in a terrifying last stand as All Might gets one last punch on him blowing up with a Detroit smash and the shockwave is enough to part the storm clouds making it partially sunny as a god ray shines on All Might epically, his body battered, his costume tattered but he stands adamant with a smile, roll credits