Elise I found was best when she is top lane because of her resemblance to Jayce and Nidalee.
Elise has the ability to jump on to a target do large amounts of damage and then get out. Also when facing big top lane bruisers like Darius and Irelia the easiet way to do damage is with % health damage abilities which on Elise can do an easy 50% of the targets health with one burst

Sorcerer's Shoes - So Elise is an AP champ and to be able to give out the damage she must have not only ability power but magic penetration which is what she will get from these boots other boots that do well with this character are Merc Treads when facing a heavy AP team or heavy CC

Nashors Tooth - Elise benefits much from this item for many reason. Scince she is a burster with more than 4 abilities she will require CDR (25% from this item) to be able to do constant burst damage, She will also gain much from the ability power scince she is an AP champ and also the attack speed to make her a better duelist, this will also scale well with her W in spider form which gives her health back for every melee attack a great replacement for this item are Athene's Unholy Grail when facing heavy AP teams

Frozen Heart - This item makes Elise very durable during laning phase which will ususally be against an AD champion. The CDR also works very well with her as in Nashors Tooth and the Passive Aura will make her a very good addition to team fights. Other items that will do the job Sunfire Cape when facing true damage champions or a team that is not very AD heavy

Rylai's Crystal Scepter - This item is a must for Elise as she will need to stay relatively close especially in spider form to be able to do lots of damage. Remember her spiderlings in late game will do around 80 damage per hit and when a champion is dueling you this will be a neccesity. Also when a champion is running away Elise doesnt have many moves to keep up other than her E in both forms. This item also packs quite a bit of AP and alot of health to make her more valuable in a team initiation. (Should not be replaced)

Abyssal Scepter - This item is great for Elise, she is not the typical AP carry which will stay in the back doing damage. Elise will get up in your face and do alot of damage, This item allows her to be more resistant to magic damage in team fights as well as to buff the damage of magic attacks done by her team by reducing the enemies magic resistance. This item will also has ability power which again is useful to an AP champion. (Should not be replaced)

Hextech Gunblade - This item is an amazing item for Elise as it will give her a great amount of life steal and spell vamp. The active for this item is great for chasing down champs which you will be doing alot and the ability power, damage, life steal, and spell vamp will make her a dominant opponent. Great replacement for this item to be able to do more magic damage Rabadon's Deathcap

In team fights never use your E in spider form to initiate this is strictly an escape, damage mitigation, or chasing ability. First start off in human form and catch one of the champions out of position with your E ability in Human form. From there quickly use your W and Q in human form on the enemy champion. At this point the enemy team should have initiated. It the enemy champiion is about to die finish them if not jump to the AP or AD carry in spider form with your Q ability and begin to zone them out of the fight. If the carry has flashed and tried to disengage out of the fight use your E ability in spider form and then use your E and Q in spider form to chase and kill them. Rinse and Repeat

Thanks guys for reading this guide and good luck in game with this you will definetly dominate if you have any requests, want me to add anything in the guide or want a more in depth guide leave it in the comments below and I will definetly help you out