It is well known there is a 10 level cap on SP gains
so fighting stuff over 10 ranks above you is useless if you want more sp


as far as everything else, especialy leves, easy mode vs challenging doesnt say much, why dont you post the ranks of the mobs you are fighting in them so that its more visible whats going on instead of star count


also keep in mind the speed you do leves adds extra sp
so if its more challenging you may actualy lose sp cuz you take longer, the secret isnt to challenge yourself but to find the way to get the most out of it

challenging yourself is your choice after all(leveling isnt supposed to challenge you as much as content is, risk vs reward in leveling isnt the same)