Malphite invinsible glitch or something

i just played a game vs malphite in a matchmade game and it was pure bs, i was playing as xin and could never do sufficient amount of dmg to in the eraly game and lost in rediculous fights but i supposed that he just to much armor and without much armor pen it was the reason, then by end of game i completed my black cleaver, last whisper, and yamatos blade or something like that, and combined had like ~85% armor pen, i know it prob scales down when u have more armor pen items but the crazy thing was even with that i suppose like 50% armor pen at mininum had to deal and even more, i was dealing like 10-50 at max to him per him even with crits and my strongest attacks, can some1 tell how this can be possible without hacking or usng some kind of glitch