Greetings, community of MobaFire!

This is my very first submission to the guides. As I haven't seen any guide about Skarner and how he should be played with ability power as his main offense, I thought I should contribute by sharing my ways of playing him.
Now, since this is my first guide submission ever, my formatting won't be that impressive and probably, I will be having to edit it many times to make sure it's just as I want it.
If there's anything you want to have answered or if you simply have a suggestion, just post me a comment and I'll make sure to reply to that as soon as I can.

When I play Skarner in a lane as ability power based, I start of with a Doran's Ring to gain some sustainability with increased HP pool and mana regeneration. Once I have some money to spend, I go straight for the Boots, because movement speed is very important early game.
To increase my sustainability even futher, I then invest in a Hextech Revolver to gain some spell vamp for my Ixtal's Impact spell. Once that is done, I get myself some tenacity with Mercury's Treads.
At this point, I pretty much have the ability power I need for a little while, instead I go for the Catalyst the Protector. That way, it increases both my health and mana, which is very much needed. Especially mana, since Ixtal's Impact is very expensive in mana. As soon as possible, I upgrade the Catalyst the Protector into the Rod of Ages.
At this point, it should be very valuable to increase health and ability power even more, which I do by investing in the Rylai's Crystal Scepter, followed by upgrading the Hextech Revolver into the Hextech Gunblade for increased spellvamp and lifesteal.

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The essence of staying alive

When you're playing at a lane as Skarner you will want to increase your durability and sustainability as soon as you can. Therefor, getting your first level at Ixtal's Impact is very important since it restores your HP when the target is attacked aditional times. Once you reach your second level it is important that you unlock Shattered Earth so that you can maximize the effect of your Ixtal's Impact. You do this by first casting Ixtal's Impact on a bunch of minions, hitting as many of them as possible, then engaging on them and casting Shattered Earth and by doing so, activating all the marks at the same time, restoring a great deal of health in one go.
By adding up your spellvamp, you also gain health upon casting Ixtal's Impact before you activate the marks. This way, your ability to stay in lane is greatly increased as long as you have Mana.

Harrasing your enemies

When you're harrasing your enemies, you obviously want to do so from a distance, so activating your Seismic Bastion and engaging with Shattered Earth is very inconvenient as it forces you to go into close combat. So instead, you stand around on a distance, repeatadly casting your Ixtal's Impact to keep them at bay. If you want to keep them from farming, you can engage with Seismic Bastion and Shattered Earth to make them fall back, taken they aren't ranged and/or you aren't at low HP. In case you are facing ranged enemies, it is important that Skarner is laning with somebody that is ranged along side with him to be able to properly harras your enemies.

Pushing/Warding a lane

In case the enemies have a jungler, make sure to not push the lane too far, so that the jungler won't be able to have a good opportunity to gank you. However, if you know where the jungler is and he's not in a position to gank your lane, then you have no worries in pushing futher if so is possible. When you can, invest in a Sight Ward or two and place them near the gank bush. Try to cover both Cash Dragon/Baron Nasher and the entrance to the enemy jungle so you can see if their jungler is on his way to your lane. This way, you can feel a lot safer while pushing your lane without placing all the responsability of warding onto your own jungler.
Ofcourse, this also saves you from any possible ganks in case the mid laner is mia.

When you're facing a teamfight, make sure you have your Impale ready, so that you can surpress whoever poses the greatest threat to the team, eg. Amumu or Xin Zhao or simply any enemy champion with strong AoE abilities. That way, the team can focus on taking out the ones that are weaker in health while you hold the one that would normally cause great harm to the party. However, don't emidiatly go and surpress him/her the first thing you do when a team fight breaks out. Try to save it for when the threat is the greatest, for an instance, if the enemy Katarina is fed, use Impale on her when she attempts to use her Death Lotus.
Make sure your team will know what you're doing when you do this so that it won't cause any confusion. If the enemy team has their strongest champion surpressed, your team should technicly be fine. However, always make sure that when you use Impale, their name ain't Gangplank (oranges fml) or that they have a Quicksilver Sash in their inventory or your Impale can easily be countered.
If you're in the midst of the battle, make sure that you spam your Shattered Earth as much as possible. Even if it doesn't cause much damage, the slow that it applies is very valuable. Not only to you, but also to any ally champion that attempts to intercept the enemies.

As Skarner main weakness when building him around ability power is his mana pool, he drains very quickly. Even with Clarity, he drops very fast if his abilities are used too frequently. It is important to feel a balance between the cooldown of your Clarity, your usage of abilities and your mana regeneration in early game. However, having Clarity in your arsenal aids you incredibly much and also your laning partner if he/she is also dependant of mana.
This summoner spell is only to make sure your Impale will land when you want it to land. Intercepting an escaping enemy champion, that tries to seek cover under a turret that you will want to pull out of there will have much more of a chance to be surpressed rather than if you'd have Flash. Since that only takes you from one spot to another and the distance isn't that great unless you're coming from the other side of a wall which you will be more likely to do if you're using Skarner as a jungler.

The Pro's:

+ Very bulky in mid/late game.
+ Strong farmer.
+ Incredible sustainability.
+ High crowd control capabilities.
+ Good mobility.
+ Looks awesome even without a skin.

The Con's:

- Low mana pool.
- Low damage output.
- Expensive item build.
- Weak at early game.
- Dependant of a laning partner with ranged attacks.
- Often mistaken for being evil, when all he wants is a hug.

All in all, Skarner is in my opinion the most awesome champion of the league. He's a challange to play. He's very versitile and can be played as a lot, which not many champions can.
Out of an AP users perspective, he has a very good ability to survive while still being able to slowly take out enemy champions. He's also a very good intercepter as his Seismic Bastion increases his speed and a charged Shattered Earth reduces the enemy champion speed.
My time so far with Skarner has been a total blast and I'm enjoying every second of it. I've been trying to play him in many ways, but in the end, the way I prefer to play him is as an offtank AP user.

I hope that this guide helps you in your ways of experimenting with champions, such as Skarner. Who knows, maybe you too will discover something new to share with everybody.