Skarner is a melee hybrid champer which excels at chasing and killing melee champions. His main source of damage is his Q skill Crystal Slash because of the low cooldown and bonus damage begining with the second usage, that's why I say that AD is more important for Skarner, besides your skills don't instantly nuke your target and you can't really use your passive properly so you must fill in with a bunch of badass AD buffed auto-attacks.

There's no reason in explaining why to chose attack speed marks (P.S. It's the passive). As a close-combat champion, Skarner, must have some defense, and for this we got the Seals(the runes, not those fat animals related to Urf), armor and mr ( every champion has skills, most based on AP, and auto-attacks, based on AD, so defensive Seals are quite usefull). I chosed AP Glyphs because all of Skarner's skills are AP based and it's good to have a short burst at early-game. ArP Quintessences: not very unusual for AD builds.

21 points in Offense, Get 1 point in cripple, 4 in alacrity and 3 in Sunder ( These ones are kind of must-haves for Skarner) but it depends on your playing style, so you can make a custom Mastery Page.

[*] Doran's Blade : A bit of everything, it gives you 100 Health which is not bad and a little boost in AD and lifesteal.
[*] Bilgewater Cutlass : When you got this item u can just go to get your blue buff to sustain your mana.
[*] Berseker's Greaves: Every champ needs boots, and the attack speed boost is not bad either.
[*] Hextech Gunblade : If you're doing very well early-game you can rush into this item (ofc after buying Boots's of speed).( If there are more than 2 hevy CCérs in the enemy team, consider buying Mercury's Treads)
[*] Guinsoo Rageblade : A common hybrid item, you can make and sustain the stacks easily with Q.
[*] Trinity Force : 150% increased damage after using a skill o.o, very good in chasing when you alternate Q with auto-attacks, and it's a must-have for skarner because all his skills got 2 effects, even the passive( i know it doesn't stack sheen effect).
[*] Nashor's Tooth : Attack Speed , AP, mana regen, cdr, a little bit of everything you need ( you can make this earlier if you're in really urgent need of mana and a lazy teammate thinks he can pass there and steal your buff and say "Oups! I was just trying to help" or "Sry, Madred's Razors" when it's obviously they w8-ed and made the last hit).
[*] Frozen Mallet : Large boost in HP, low in AD, mut the main reason why we're making this item is the passive, 100% chance to slow a target for idk what percent, it synergies well with his Q in chases.

Pros
- great chaser
- element of surprise (more Dmg at your second cast of Q), if your target didn't see the champion spotlight or it's just noob.( or it's a minion they wont know what hit them^^).
- extremely funny ultimate + u can spam Q all the way to slow him after the effect ends and/or yous Crystaline Exoskeleton to move him even further ).

Cons
- mana hungry early game untill you gain you're blue buff
- Q stacks with a portion of your bonus AD so you dont really deal damage with your 1st Crystal Slash cast untill you get your pickaxe
- often CC'd in team fights to to his ultimate that can disable they're carry
- IT'S TOO SMAAAAALL!! at sneak peak they showed us a giant scorpion destroying villages ( maybe a hobbit village:-?) and now we get this tiny little bug -.-', or maybe all League of Legends Champions Are 10 times the size of a normal human hmm...

I get Flash for a quick escape way or reduce gap skill, and exhaust against powerfull carries when the ulti is on CD or in chasing someone when i'm out of mana ( you can use Q>Crystaline Exoskeleton>Impale>Run like there's no tomorrow!!!but not too fast or you'll lose your target>Q again to slow>Exhaust>Hextech Gunblade to chase down high HP enemies or CC them untill your team comes and helps you)

Your role in a team fight is to focus down the carry, catch him with your ultimate then use your aoe skills to heart all their team.
If you get late in a team fight and the enemy chases your teammates ( and you think you went get killed) just go at them and slow them with your Q, when they try to run catch them and bring them back with Impale to FINISH THEM!!!

So, the conclussion is that Skarner is a little bit tricky champion, but once you get used to it he can become a BEAST!

As my friend Porky would say "That's all folks!"
P.S. When an enemy is ready to escape and you flash in and ulti him back to your team try an evli laugh like "muahahahahahah>:) :->~~" in All chat, this really pisses him off, who knows maybe a rage quit follows :-"