no damage items... says the malphite had Runic Echoes...
Runic Echoes stats: 60 AP, 7% MS and a passive that deals 60 (+10% AP) damage when an ability hits if he has max charge... and deadman's also deals 100 bonus damage if it's also fully charged if you get hit by an auto... and then you look at Malphite's kit
Q: 70/120/170/220/270 (+60% AP) magic damage
W: 15/30/45/60/75 (+10% AP) (+15% armor) bonus physical damage for 6 seconds with each auto attack... while also passively increasing armor by 10/15/20/25/30%
E: 60/95/130/165/200 (+20% AP) (+40% armor) magic damage
R: 200/300/400 (+100% AP) magic damage
i have no idea how he was distributing his skill points and what his masteries were but if he actually did as much damage as you say he did then that tells me that you had no actual defensive stats... got hit by his R then his Q which essentially steals MS from you and gives it to him, he got on top of you and activated his W and punched your face in while using his E in between...
it doesn't matter if you are 9999999/0 or 0/99999999, if you build no defensive stats you aren't going to magically take less damage just because someone is not doing so great