Well, let me put it another way.
In the cross-ranking battles he already had, it was his abnormal defence and abnormal attack that allowed him to overcome his battles, take for examples when he let the harpies hit him to check out how much leftover damage was he took after 4 layers blablabla..
Was there any mention of him reflecting back 10% of damage taken having any effect whatsoever on the opponent?
Putting aside that it isn't clear if the reflect is based on "damage taken" or "attacking power" .
Besides, that, my random example was a 100% reflect back for 100% of their attack, but that was only 10% of their HP. if MC has 1000 HP and 100% and opponent strike for 1000 damage and has 10000hp, result is MC dead and opponent at 9000 hp!
And the cross-ranking battles are the bread and butter of cultivation novels, so the MC real opponents WILL have magnitudes higher hp and attack power than MC.