Hi, my 2 cents and to take with a grain of salt, since I never reached diamond.
Other than malphite, do you think ap is the best build against some champions in particular? Heimerdinger comes to my mind to kill turrets aoe. Also, have you tried rushing dusk and dawn + gunblade? For me, it feels the same damage-wise, but with better sustain and more usefulness while w is on cd.
Last thing, what what do you think about grasp to make some hard matchups easier? like riven/pantheon/teemo. Also with riven, when a bit ahead it feels better to rush essence reaver and kill her before she can do her full combo (which doesn't seem to care much about my armor or sustain. Thanks for reading!