In most cases I go for a nearly pure tank spec on Elise. 9/21 defense masteries. I either start with the +Health/mana thing and a pot or cloth armor and pots, depending on jungler a ward too.
Against AD tops I get that new item (armor + AP, + extra armor and AP on minions kills (builds into Zhonyas anyways). Mostly Sorc Shoes, Giant Belt, Glacial, Warmog, Frozen Heart (or Gauntlet if anyone else wants Frozen Heart), Abyssal or Banshees depending on enemy set-up, Zhonyas. If I feel I'm tanky enough I could get Liandry's for more Ap and MPen.
But in general you do enough damage and can get tanky as fuck.