1) what's her skill combo? I'm assuming human form e,q,w --> spider form q, w and ignite?
2) How do you build her? I did sorcerer--> haunting guise--> liandry--> abyssal scepter--> warmog
1. Pretty much yes. Human E is optional, naturally, but if you can hit the stun there's honestly no reason not to. In spider form, Q is first because you want that cooldown ready again ASAP in the more prolonged fights, and because Q also causes your spiderlings to leap at the target, putting them into position for optimal damage from W.
Note that you don't necessarily have to engage to the death just because you went spider form. You can perform your combo then back off after the spider form Q->W and let your spiderlings deal a bit more damage while you scuttle away. Your stun will usually last long enough to get that combo off and back away without much retaliation. Typically, though, you just want to use human form Q to whittle their hp down to below half, and then combo them to finish them off, since using spider Q to add to the harass can become predictable and can screw you sooner or later.
2. You have the right idea there, though you don't necessarily have to rush Liandry. Boots and Guise are enough for laning phase, then it may pay to either buy a Chainmail or a Negatron depending on who you're against, then decide on whether you want to continue Liandry or upgrade the Chainmail / Negatron first. You pretty much go for the Liandry if you're stomping or you're going against someone stacking health.
Personally I like either Frozen Fist or Banner of Command for the Chainmail, and Abyssal for the Negatron. Frozen Fist is much better for boosting your own damage, while Banner of Command boosts your spiderlings damage when you're against CC heavy opponents. Frozen Heart is also a good choice if you're up against a team that's heavy on autoattacking champs.
But yeah, your build is pretty good. Replace Warmogs with "Situational" though. Warmogs can still be a good choice but depending on the game you may want something else there instead. For Elise's damage output though yeah it's mpen -> CDR -> AP.