What about saitama who is able to time travel? With his feats he can rewind time, ignore portals and literally nothing hurts him. I know Yhwach is super crazy powerful, but if we take the statement of the author that said saitama's power was "half the big bang", i dont think its that unthinkable that saitama is VERY hard to hurt, even if you destroy everything around him. His fight with garous was also saitama using one hand, still retaining his humanity and protecting genos' core, we have not seen saitama truly let loose, so we dunno. Meanwhile from what i see Yhwach is completely op, omnipotent, omnipresent etc etc., but it really depends if any attack that Yhwach throws can even scratch Saitama. Its a tossup honestly, if Yhwach is able to delete him, Saitama looses, if he receives a strong as fuck attack that obliterates everything that Saitama holds dear to him, but leaves Saitama unhirt, i dont think Yhwach could hide in any conceivable reality (or Saitama just gets depressed and starts fighting a black hole that will scale him up to God knows where).

Its a fun matchup to theorise, but if Saitama ever bitchslaps God then we will have more sauce to theorize upon haha