combo is really up to you, but typically you want to aim for CDR + tankiness with AP dabbled in where you can. Mix and match and experiment. Just remember to stick to the core. Elise scales off enemy having no MR more than anything else because of her % damages.
What do you do when faced with two choices? Simply toss a coin. It works because in that brief time that the coin is in the air you know what you're hoping for.
Pretend she's Cho'Gath or Amumu or something, build accordingly. Boots, Abyssal, Sunfire, Frozen Heart, Rylai's, tanky AP things. Since you're likely to be fighting AD champs top, focus more on armor-oriented AP things.