Elise is one of the best and if not THE best anti tank in the game with primarly % of health, true dmg. Almost every game the tank go top so thats why elise is such a good top laner.

Bad things about her = Very easy to kill early game.
Easy to loose your lane if going to agressive.
Shes screwed if som1 comes to gank her early on.

Good things about her = Impossible to kill lategame
Awesome lategame teamfights, great assasin.
No tanks can stop her becouse of her % of health dmg.

If jungling or counter jungling make sure to check the buff with your E to see if som1 from the enemy team is taking it.

Use your W to take out creep wawes, if you run out of mana simply go into spider form.

In teamfight you should always E into their adc and take him out, your tankyness will make it impossible for the other enemies to kill you.