Skarner in my opinion is a borderline OP champion, being an amazingly quick Jungler and a fantastic ganker post-6 he tears champions apart and can carry games. In my opinion, his best trait lategame is his ability to flash in, grab their best champ and let your team kill them. Forcing a 4v5 which is a lot easier to win. This is how i play and win with him.

For the record my ranked stats with him are 8 wins to 3 losses (im PocketMarsh if you wanna check me out)

The Reds (attack speed) are a great help to Skarner because of his passive which helps with clearing the jungle, ganking to get his Crystal Slash up quicker, and ultimately make you a better champion

The Yellows (armour) are for jungling, nothing more, nothing less. Standard Jungle runes

The Blues (CDR/Level) are for endgame when you want to be focusing on perma slowing/damaging the enemy team, and getting those cooldowns lower helps a lot.

The Quints (Attack Speed) Look to reds for info.

Defensive Masteries i feel are the best, because there is no real need to go 21 in Utility and neither are the stats that great in Offesive....

I use these items to give me a good mix of damage and balanced defenses. Ideal for an offtank. Wit's End i feel is an awesome item for Skarner because of the Attack speed granted from his W and the extra bonus of having AS for his passive. The MR is also an added bonus.

Flash+Ultimate is what makes Skarner boss. I see no substitute for these summoners unless you are top lane in which case i would get Teleport. NEVER TAKE ANYTHING DIFFERENT FOR JUNGLE SKARNER!

Get a leash and then drop Blue with your Crystal Slash.

Move on to wolves, using Fracture to heal up when you can

Next to wraiths, keeping up health pots the entire time

4thly to Red buff. Drop it with Crystal Slash level 2 and Fracture, Smite should be back up

Lastly to Double Golems to reach level 4

You are now level 4 and it is time to gank! Look for a lane able and use Red+Crystal slash to take them down.