Focus interlocking using both human and spider form's Qs to finish minions.
Use spider form's Q to finish cannon's since its damage is gonna be higher.

Since you have doran's ring, if you are full on mana and Q an enemy champion in spider form, it is gonna heal you a small amount, so give preference to start exchanges with your spider form to keep your mana up for that.

Whenever your Grasp of the Undying is up, do a human basic attack and transform into spider while the basic attack is in the air so that you gain bonus max health from Grasp as a melee with a ranged attack.

If you are Jungler with Conqueror instead of Grasp, just ignore all mentions of Grasp in this guide, but the build is the same.
You just abuse your Frenzy W in spider form to reach 12 conqueror stacks in a very short amount of time, which also increases your passive healing.

Your first objective is to get from 1100 gold to 1400 gold. The most ideal case being 1800 gold.

On your firt back base, get a Sheen, Boots and if possible a Ruby Crystal, that will increase both the damage from your spider Q with sheen and human Q with health.

If you have less than 1000, grab sheen first.

Rush the Divine Sunderer as much as you can, starting with sheen and bonus health crystals to increase your human form's Q damage as well.

Then, start your Nashor's Tooth by the Recurve Bow, to increase your base attack speed that is further increased on spider form's Frenzy(W) and your spider form's Q bite's damage.

Nashor's Tooth will also increase your human form's basic skills and allow you some poke with Volatile Spiderling to even further zoning your enemies while also making farming easier on human form basic attacks.

You actually have a choice of going either Titanic Hydra or Ravenous Hydra.
Ravenous Hydra will increase your healing by a LOT and make your human form's Volatile Spiderling have even more damage.

Your human form's basic attack will already have a considerable damage by this point while also having splash area damage, which allows you to farm the whole lane or harpies in a matter of seconds.

You can also swap your Hydra for a Serpent's Fang if your enemies have too much barrier and you want some anti-barrier.

Demonic Embrace will make not only your human form's skills burn enemies for percentage health, allowing you better poke and more AP for skills and Nashor by turning all the health you built so far into AP, but will make your spiderlings and Venonous Bite to also cause burn, making sure even if enemies escape from you, they still might die by your poison, while also further increasing your max health which in turn also increases your human form's Q.

By then, you will have the surprise engage potential of an assassin with the damage of a melee champion, ranged fighting potential of a mid-to-high level mage or adc while having a bruiser's max life and tankyness making you an extreme threat on the fieldn and very, very hard to deal with.

Ironic that the Divine Spider would have a Demonic Embrace's poison.

The last time leaves open multiple possible items depending on the situation, being a very flexible closure.
If the enemy team has too much magical damage, you can go for either the Abyssal Mask or Spirit Visage, with the Mask giving you more damage after the Cocoon and the Visage increasing all your healing.

If they have a lot of Physical Damage, you go for the Thornmail or Deadman's Plate.
Thorn if they have some healing and Deadman's if you want your bite to hurt even more and even cause some slow.

You can also choose to build a Warmog, so that your health is always full and to be able to withstand even more damage by yourself, which due to the Demonic Embrace, will also give you some AP.

A Cosmic Drive, to further increase your AP and Ability Haste, making your bite come up even earlier, and since it also gives you health, you will be able to withstand more and that health also gives you more AP on Demonic Embrace.

Or even a Rylai's Crystal Scepter if you want to make sure your enemies wont be able to run from you anymore.

Surprise Engage Over Wall:
Human Form's E > R(Spider) > Rappel E > AA > Q > AA > W > AA AA AA AA > R(Human) > Q > W > AA > E > Repeat Spider form

Lane Burst:
Spider Form's Q > AA > W > AA AA AA > (If chasing, Rappel >) R(Human) > E > AA > W > Q > AA > R(Spider) > Repeat

Mid-range Poke:
Human Form's Q > AA > W > E > R(Spider) > Q > AA > W > AA AA AA > Rappel E out on a minion

As a laner, after you finish Nashor you can extremely easily and quickly take jungle camps, so you can leave advanced wards near blue and red enemy camps and constantly steal them or steal wolfs and harpies to keep the enemy jungler weak whenever you know which side of the map he is.

If you see your jungler taking the Scuttle Crab on the opposite side of the map and your side's Scuttle is up, you can also very quickly take Scuttle using you cocoon to break its barrier, just be careful no to be picked off while doing that.

If engaging, try to always keep your cocoon for after your spider form's rappel since it makes it almost impossible to escape the cocoon by using it immediately transforming after rappel, which also grants the Volatile Spider 100% chance to hit and allows you 2 Divine Sunderer empowered basic attacks, 1 immediately after cocoon, and one after Q.
Sequence:
Spider Form's E(rappel) > R(Human) > E(Cocoon) > AA > W > AA > Q > AA > R(Spider) > Q > AA > W > AA AA AA

Also remember to abuse your spider form's W to take down turrents.
You can also use your ult change form and human form's W to trigger Divine Sunderer for extra damage to the turrents.

If possible, try to get the enemy's red buff, since the damage per second makes your spider Frenzy's damage ramp up even more.

Always ward the herald since you can make it very hard for enemies to take it with cocoon and Volatile Spiderling and you make your own jungler job a lot easier to help him take it.

If your jungler is someone with 2 or more stuns, try to wait for their first stun to hit before using your cocoon, to engage on a chain CC that basically secures a kill, starting your engage in human form instead of in spider form as you would if alone.

If you are against enemies that have a lot of healing, swap out your Rylai's for a Morellonomicon, making an Oblivion Orb as soon as you finish Nashor's Tooth before going for Hydra.