Garbage Player Looking for Matchup Help

Hi, I'm a Gold 5 scrub that just wants some tips about Malphite because I've really been struggling lately. I don't understand how I can have 500+ armor and get shredded by 1 armor pen item. I frequently get up against full AD teams and build pure armor, but still get shredded and bursted. I just played against a team where the Garen had , , , and . I had , , , , and . How could I not 1v1 this guy. I know he has cleaver but is it really that powerful? What am I supposed to do if we lose team fights and I can't even 1v1 when I am super fed? And how should I build against people like Vayne or Master Yi with built in armor shred? Is the character that bad, or am I just terrible? What is the optimal rune page to deal with this, cause Grasp of the Undying seemed to be really good in lane but useless after. I'll take any tips I can get at this point, cause I love the champ but get depressed as hell seeing him lose to AD when that's supposed to be the thing he stomps. Thanks for taking the time to read this and I hope you all enjoy your games more than I have. :(