Spider FormSpider Form and Spider Form abilities do not cost mana and can be prioritized when you are trying to conserve mana.
Use Spider FormSpider Form when escaping, as the bonus armor and magic resist will make you tankier and you can use Venomous BiteVenomous Bite as a last resort on minions or monsters and Rappel will make you untargetable. Being in this form also grants you 10 bonus unlisted movement speed.
Elise is a deceptively effective pusher & counter jungler due to her spiderlings. Use Spider FormSpider Form to take enemy jungle camps, towers, and other structures when the opportunity presents itself.
Lich Bane is a very effective item for Elise, as it grants some more magic resist and a huge amount of damage after every spell.
Zhonya's Hourglass is another defensive item which gives you more damage. Also its active ability can save you in difficult situations, synergizing well with the short time of disappearance from your Rappel.
Rappel will only move Elise straight up and down unless she can descend upon an enemy unit.
Rappel has a long cooldown and Elise is vulnerable after she has used it.
Rappel can be cancelled if you use hard crowd control on her (such as for example the knock-up from Jarvan IV's Dragon Strike) while she's on her rising animation, and it will cause her to immediately descend upon rising.
When Elise targets you with Rappel, she can be lured into turret range or other dangerous areas by moving there when she starts to go up, as she cannot cancel the ability once cast. A quick teleport or dash is the most effective way to do this.