Here is how I play Fizz, to make him a high bust damage, capable of knocking out squishier champions within a few seconds before they can even react, or escape. It's also great against helping other take out tanks. This makes him a great assassin, and excellent at chasing down weak champions. It also keeps him alive and kickin' in long fights, provided you don't get focused.

Mind you this is my first time making a build for a champion, so please excuse anything that I missed, or did wrong, and please feel free to correct me.

For the Runes, I build with Ability Power and cooldown reduction. This helps Fizz be strong early game, and have his abilities up quicker. This makes him deadly in fights because of his ability to dish out a lot of damage with abilities and make him able to chase down and escape from enemy champions.

I build Fizz offensively, making sure to max on Ability Power and damage. This helps Fizz be dangerous from the get go, and if you can start feeding on enemies and get his build going early on. It's pretty straight forward, and can be tweaked to your liking.

I like to start off with building the Rod of Ages. Start with the Ruby Crystal, then the Sapphire Crystal. This will give you health and mana. Then grab the Catalyst, and this will keep you out in the field every time you level up, you get the health and mana regeneration. This also allows you to always have mana to use. Then you can finish it off, and you'll be getting pretty dangerous.

Then the Sorcerers Boots, these are a given and pretty easily understood.

Rylai's Crystal Scepter is next. This gives you a health and AP boost, as well as it's Passive is great for chasing and keeping enemy champions in range for your abilities. I like the health bonus because it helps keep you alive. I start with the Giants Belt, since it is the most expensive. Then grab the rest of it.

Now is where I start switching to Ability Power items. If you're facing a team with a lot of high damage champions, or you feel like you're getting crushed before you can do much, get the Thornmail, this will help you when you get focused. If not go for Rabadon's Deathcap.

I follow this with Zhonya's Hourglass. I like the Ability Power, and the armor. The active ability is nice if your pals show up, and you're about to die, it can help you survive long enough to escape.

And if I've ended up skipping Thornmail, I get the Void Staff, for the additional Magic Penetration. It makes Fizz pretty tough to kill and hard to fight with stacking Magic Resist considering you have 20% from the Boots, and 10% from your mastery. This puts you at 70%, which is tough to beat.

I start with Seastone Trident, because it makes farming mobs early easier. Most fights early game you can run up to them, or surprise them from the bushes and get the kill with the DoT.

I then move to Urchin Strike. This when paired with W, can be deadly when champions try to run.

Then move to Playful/Trickster. Now you'll be able to fight well, and escape easily.

I build them all in this order throughout the game till 18, picking up Chum The Waters whenever I can, and keeping with this order. It keeps Fizz balanced, and deadly all around.

I use Heal and Ignite.

I get a lot of flack for using heal as a Level 30 summoner. However, in team fights, and for surviving when laning or turret diving, it's invaluable. It keeps me and my fellow laning partner alive, and allows me to save them and keeps the enemy from getting kills, and maximizing our ability to feed.

Ignite is great for champions with lifesteal, and healing abilities like Volibear's Passive. This allows you to damage them and catch them with E and Q, or R if you have it up, or haven't used it. Plus you can use it to harass enemies.

Pros:

High damage with abilities
Survivability
Harassment and escaping
Capable of laning mid

Cons:
Damage against turrets minimal
Tanks can batter you down in a long fight
Ranged Champions with lots of Attack Damage can burn you before you catch them.

Farming is hard until you have Rabadon's Deathcap, seeing as you have to either burn abilities or auto attack which can be slow. Your E can hit multiple enemies and kill them all instantly once you have the deathcap, and this maximizes your gold earning abilities. I generally use either W or E to farm.

I generally wait for my fellow champions to go into a fight, then run out of the bushes, and start burning abilities on the enemy, focusing the squishiest champions first. I generally come up at the back of the pack in a team fight to avoid being focused, slip into the fight, and start picking off weaker champions or ones that are running away.

Keep in mind that if you find yourself getting focused, you can either get Thornmail, or escape using E and Q by targeting enemies that are behind you, or closer to your nearest turret, or area of safety.

The golem buff [blue], is wonderful for Fizz due to the Cooldown Reduction it provides. You can use abilities almost instantly after using them once you're at 18.

So, all in all this is my high ability power build for Fizz. It address his squishiness, and builds on his ability to be in and out of fights quickly, and making him a force to be reckoned with.

Please feel free to offer any advice or constructive criticism. This is my first time making a guide for a champion, so if you feel like I left anything out or have any questions, please comment, and I'll do my best to address it as quickly as I can, and add in anything I missed.

Ian