This is the main build I use for Skarner when I jungle with him. Instead of a full tank build, I substituted some of his tankiness to add more attack damage. His AD will usually get to 200 or above while still having over 3000 health and a considerable amount of armor and magic resist, depending on what the enemy team looks like.

For the runes, they're pretty standard for Skarner, I think. Armor and MR yellows and blues, movement Quints and flat AD reds.

The masteries are off for Skarner. Most would go 0/21/9 but I go 12/15/3 taking only the physical damage route in offense, only 3 points in Mana Regen in utility and the extra health, health regen, aromor & MR and Bladed armor for jungle monsters. Until you get Triforce Skarner can be mana hungry outside of the jungle and in team fights, so the 3 points in mana regen does help.

The items will change for you depending on the enemy team's damage output. Most of the time I'll take Mercury Treads for the tenacity but if their team had little CC then go with Ninja Tabi. If the enemy team has a ton on really hard CC then I'll replace Sunfire Cape with Mercurial Scimitar to remove all debuffs from me, along with the extra AD and MR. Most of the items are situational, but you'll always want to get Trinity Force, Randuin's Omen and Blade of the Ruined King. I can usually finish Triforce about 10-13 minutes into the game if it's going well.

Not gonna lie, Skarner's 'E' ability isn't that good at all. Just level up his 'Q' whenever possible and get his 'W' at level 2. Don't get his 'E' unless there's nothing left to level. And of course, get your ultimate at the available levels.

Smite and Flash. Not much else to say there. I guess you could take Ghost instead of flash if you want to be even more fast since Skarner is a pretty quick champion already, but I always take Flash.

PROS: Hits for high damage mid-late game, is still very tanky for such high damage, has high movement speed and a consistent slow on his 'Q', can peel off enemies from a team fight with his ultimate.

CONS: Has mediocre ganks before level 6, only method of escape is his 'W', besides Flash.

Always get your team to leash the Blue Buff for you, otherwise you'll take a lot of damage as Skarner is not too tanky at level 1. Then do wolves, Wraiths, Golems, then Red Buff. At this point, check to see if any lanes are gankable. If not, continue killing jungle creeps.

When ganking, always come into the lane with your 'W' activated. The increased movement speed will give the enemy less time to react and you'll get your ultimate off easier.
In team fights, always use your ultimate on the highest damage dealer to prevent them from killing anyone and your team can kill them quite quickly. Some other good champions to use your ultimate on would be champions who have high mobility such as Ahri, Akali and Fizz.