I tried to play Elise many ways but I found this as the best, because she deals % life damage and starting with defensive items can help you making first blood really easy.
I'm sorry for my not perfect english, so if someone would help me improving/writing this guide, I'd be grat.
She's is built up like a normal offtank, I got inspired by offtank teemo build so I tried it on her... and BUM!

About runes i prefered using Armor Penetration + Defensive Runes, because I find them useful in mid/late game. You can play also her with red and quintessences of AD

I chose this mastery tree because i built her to survive in teamfight, in fact you can observe that usually she get pwned, but basically she's a damage character and so her mastery must be for an aggressive play.

I gank in this way E-Q-W in the spider form so I jump on the enemy and start attacking him, then when he start fleeing transform in human and use E on him (you will certainly hit him because you are near him) so: E-Q-W