I tried out a lot of builds with Skarner but none of them brought out his full potential to be a hard/tanky carry while still being able to dish out a ton of damage and walk out of a fight unscathed (usually). So after experimenting with items for a while this build gets the most out
Skarners abilities and still allows him to be incredibly tanky late game.

AD= Attack Damage
AS= Attack Speed
ADR= Attack Damage Resistance
AP= Ability Power
Ap= armor penetration
MD= Magic Damage
MP= Magic Penetration
MR= Magic Resistance
DoT= Damage Over Time
CDR= Cool down reduction
HR= Health Regen
Mr= Mana Regen
CD= Cool Down
CE= Crystaline Exoskeleton
CS= Crystal Slash

Pros

Has an easy spam slow
Heal
A shield that gives him a move/attack speed buff (epic chaser)
His ult is a great deny to just about anything for the enemy team
CD passive based on hits XD

Cons

Can sometimes be squishy early
once u loose your CE u just lost a great advantage

For runes I got flat Ap marks and quins. Killer why dont u use MP marks and quins instead? easy: Skarner even though his attacks are AP based he still relies on physical damage to get most of the job done. Armor per level Seals, and CDR per level glyphs.

I go for a 9-0-21 mastery build in order to grab some AP, CDR, and the MP in Offense. While getting Regen (mana/health), a bit more mana, lower CD on flash, move faster to chase, and even more CDR just to spam that Q even more

Flash for escape, and ignite to make sure that u finish them for sure, also nice against healers like Sona, and Soraka, or crazy HR like Mundo

I start of with a Dorans shield because i get Health/Armor/HR what more can u ask for lane sustainibility. After u get Boots rush for sheen. Trust me sheen is basically GOD with your q as the CD for the q scales perfectly with sheen 2 sec CD. Get sorcerer's for that MP i promised, Phage for health/AD/Slow, Zeal for Attack/Move speed and criticals (when u crit with sheen you do about 400 damage). Trinty Force is your bread and butter for Skarner it gives you just about everything to dominate your lane. Then get hextech revolver cause you are AP, then vampire for physical bilgewater cutlass, and finaly Hextech Gunblade

If the enemy Team is AP heavy (Brand, Malzahaar, AP Shaco) get:

Hexdrinker (MR and Magic shield that absorbs 200 MD if the attack brings u below 10% health)
Banshee Veil (Health/Mana/MR/Ability Blocker)
Force of Nature (MR/HR per second) or
Wits End (MR/AS/MR per hit)

If the enemy is AD/Crit Heavy (Master Yi, Tryndamere, Gangplank) get:
[list=2]
Thornmail (AR/and attack dealt to you are returned in some MD)
Randuins Omen (AR/HR/Health/Slow based on enemy armor)
Sunfire Cape (AR/Health/DoT) or
Banshee Veil (Health/Mana/Ability Blocker)
[/list]
If dealing with a Tank get:
[list=2]
Madred's Bloodrazor (AD/AS/Shreds enemy Health with MD)
Last Whisper (AD/Ap)
Void Staff (MP/AP)
[/list]

CE for initiate, Spam your Q every chance you get, and when your low use fracture to get back in, and when they run Impale the sucker. The best way to use Impale is not to think about getting a kill its about GUARANTEE A KILL for you or your team.

I hope you like this guide and i hope you'll be kicking some newbie butt out there!!!