-you don't need the cinder for waveclear, that's overkill on diana, don't do that
-if you want more armor you should go either for GA or zhonya, your barrier should be enough to make you survive, even if the slow is somewhat nice it's better to build a rilay, but this way there is another problem: you are supposed to shot targets, and follow them with your ult refreshes with Q, if you go that build you won't do damage and you won't tank, you will rely on the discrepancy between you and the enemy team, not on your build
-mercs is nice, just build it situationally tho, i wouldn't recommend it
-you should build void staff as most inexperienced players don't realize how good it is, basically in your eyes people have -35% hp
-also you should build lich bane as it synergizes too good with your kit
a build i suggest is the following
>
>
build all 3 as starting items

situational

And push after lv7