I've been playing Poppy ever since I first started to play League of Legends, but I never had the time to rework my guide. Previously my guide included two parts, a DPS build, and a AP build. After playing Poppy for quite some time, I've come to the conclusion that AP is the way to go for massive damage in seconds.

This build is what you want if you want to do a lot of focused damage in seconds with Poppy. Although many players play her DPS with Trinity Force, I like to play differently and build for more nuking power, as opposed to only physical damage, making Poppy more spell-oriented rather than rely on solely melee hits for damage. Poppy is a very overlooked champion, I'm not sure why.

Valiant Fighter
Any damage dealt to Poppy that exceeds 10% of her current health is reduced by 50%. This does not reduce damage from structures.

-This passive is really useful when you're low on health and running away, or being chased by enemies.

Devastating Blow
Poppy crushes her opponent, dealing attack damage (+60% of ability power) plus 20 / 40 / 60 / 80 / 100 and 8% of her target's maximum health in bonus damage. The bonus damage dealt cannot exceed 75 / 150 / 225 / 300 / 375.

Cooldown 8 / 7 / 6 / 5 / 4 seconds | Cost 55 mana | Range 600

-This spell is your damage dealer, your primary output of damage. This skill is your bread and butter for Poppy.

Paragon of Demacia
Passive: Upon being hit or attacking an enemy, Poppy's armor and damage are increased by 1.5 / 2 / 2.5 / 3 / 3.5 for 5 seconds. This effect can stack 10 times.

Active: Poppy gains max stacks of Paragon of Demacia and her movement speed is increased by 17 / 19 / 21 / 23 / 25% for 5 seconds.

Cooldown 12 seconds | Cost 70 / 75 / 80 / 85 / 90 mana | Range 600

-This spell's passive is good for running away, or charging in. The active is great for both as well, with the movement speed boost.

Heroic Charge
Poppy charges at an enemy and carries them for a short distance. The initial impact deals 50 / 75 / 100 / 125 / 150 (+40% of ability power) magic damage. If they collide with terrain, her target takes 75 / 125 / 175 / 225 / 275 (+40% of ability power) magic damage and will be stunned for 1.5 / 1.5 / 1.5 / 1.5 / 1.5 seconds.

Cooldown 12 / 11 / 10 / 9 / 8 seconds | Cost 60 / 65 / 70 / 75 / 80 mana | Range 525

-This spell is your initiator spell, the spell you cast before all others. (most of the time)

Diplomatic Immunity
For 6 seconds, Poppy is immune to any damage and abilities from enemies other than her target.

In addition, Poppy's attacks deal 20 / 30 / 40% increased damage to the marked target.

Cooldown 140 / 120 / 100 seconds | Cost 100 mana | Range 900

-This is arguably one of the most 'OP' ultimates in the game, just because it makes you invulnerable, and do extra damage to a target. This spell can be used to kill enemy champions at their own turret, or, you can charge into an entire team and mark one enemy to kill, since the ultimate makes it so you only take damage from the person that you have marked.

R> Q> E> W

You will want this skilling order/priority. Get Heroic Charge first, and from there on, level Devastating Blow and then Heroic Charge back and forth. But you will want one point in Paragon of Demacia at level four, as it is always good to have one point in your unfavored spell at level four. And from there on, level your ultimate, Diplomatic Immunity whenever available.

You want your spell combo to annihilate your enemies to be to:

Heroic Charge to their location
Deathfire Grasp on them to do 30% damage equal to their current health (if you have it)
Devastating Blow on them if they are not dead, and then repeat this, minus the Deathfire Grasp if they are not dead

You will want to throw in Paragon of Demacia in there somewhere, it doesn't make much of a difference, though. Also use your ultimate, Diplomatic Immunity when the target is surrounded by multiple enemy teammates.

Starting item:

You will want to start with Sapphire Crystal almost always, as it gives extra mana that you require to harass enemies early game, and will build into your Lich Bane. However, if you really need it, Doran's Ring or Doran's Shield are both viable.

Core build:

Sorcerer's Shoes are for the magical penetration (of course), I generally wouldn't get any other kind of boot when going AP unless you REALLY need it, because the magic penetration boosts your damage output by a lot. Lich Bane is your primary item that you will want. It is your damage dealer, because it boosts physical damage by your amount of AP after a spell. Deathfire Grasp is your initiator item, as you will be casting the active that it has first in every fight you do. Void Staff is extra AP and magic penetration, what more could you need? This boosts your damage output to amazing levels.

Late items:

These items are pretty much self explanatory, massive AP bonus, which will increase your damage. Zhonya's Hourglass will allow you to run straight into a cluster of the enemy team and use the active, thus making them focus you, and your team will rush in and pound them to oblivion.

Your role as Poppy is to take out the enemy team's most successful/damaging person. You are an anti-carry, meant to stop their carry from being successful.

Exhaust and Ghost is what most Poppy players go for. It is great for the bonus speed if you need to run, or if you need to chase. The Exhaust pretty much makes their carry useless when you use it on them.

Other viable spells:

Ignite for executions/constant damage.

Flash chasing/running.

Clarity keeping your mana up, staying in lane longer.

Cleanse if you are focused and hit by heavy stuns and crowd control.

21/0/9 is what you should go for if you are a melee based, DPS Poppy. However, an AP poppy should use 9/0/21 for cooldowns.

Magic penetration is what you should be going for the most as an AP Poppy, and that is why I got magic penetration marks and quintessences. Dodge chance or mana regen Seals are usually the most favored ones for Poppy. For Glyphs, I recommend cooldown reduction, whether you are AP, or DPS, or whatever you are, it is good for keeping your Poppy build well-rounded.

Work with your team to nuke their carry in team fights. However, when laning, you will want to work with your lane mate to use your skills to pin your enemies against a jungle wall and stun them, and then nuke them. Repeat this until your enemies have fallen.

By now you should have a good idea of how to play Poppy as a caster. I hope my build has helped you become successful at Poppy, or to get a general idea of what Poppy does. Everyone should realize that Poppy is truly a force to be reckoned with.