Pros
1. Surprisingly, Has Naturally Fast Movement Speed
2. Great Farmer with Guinsoo's Rageblade and Nashor's Tooth
3. Great Lane Pusher
3. Great Ganker

Cons
1. Mana Heavy Early Game
2. Not the Strongest Champion Early Game
3. Needs Certain Items to Shine

Greater Mark of Desolation To Synergize with Trinity Force
Greater Seal of Scaling Mana Regeneration Because He Constantly Uses Mana
Greater Glyph of Scaling Mana Regeneration He Needs More!
Greater Quintessence of Movement Speed He is Already Fast Enough! Why Get!? I'll Explain in the Items Explanation Chapter Below

9/21/0

Because Skarner is a close-combat melee champion, he will need some defense. Problem solved with masteries. However, he is always mana hungry. Therefore, I solved his mana problem with Greater Seal of Scaling Mana Regeneration and Greater Glyph of Scaling Mana Regeneration.

Max Shattered Earth first. Even though Ixtal's Impact may initially do pretty good burst damage, it only scales with AP and your main damaging ability should be Shattered Earth. Max Seismic Bastion next, because it increases your movement speed and attack speed as well as giving you a shield. More movement speed will synergize with Boots of Swiftness and Trinity Force, and more attack speed will synergize with Guinsoo's Rageblade and Nashor's Tooth. Max Ixtal's Impact last, but you MUST NOT neglect this skill. When you engage in teamfights, you should be using every skill whenever it's up.

How to Use 'Em!
Knowing how to use Impale will depend on how much you practice. As for other skills, you must use every single one of them whenever they're up like I mentioned. There is no such a thing as a rotation. Use Seismic Bastion to escape, catch up to an enemy, or soak up a harmful spell. Use this skill to gank, use it right before you Impale so you can make them run more. Use Ixtal's Impact to kill an enemy champion who's about to get away with 1 health! Use it to heal yourself. You will mostly be using Shattered Earth in teamfights, so make sure you spam that spell. Use it to slow, use it while an enemy champion is Impaled so you can slow him/her right after Impale ends.

Ignite and Flash for obvious reasons.

Ignite will help you out early game as well as giving you some extra dps late game. Flash is just always convenient.

You don't need Ghost, because with Greater Quintessence of Movement Speed, Boots of Swiftness, Trinity Force, and Seismic Bastion, you will have over 510 move speed!! Even if you were to use Ghost, there would be a diminishing return on your move speed increment, meaning you wouldn't notice any difference even if you used Ghost.

You don't need Exhaust, because you have so much move speed to catch up to your victim and slow them with your Shattered Earth. Having trouble slowing enemies with Shattered Earth right after Impale ends? Practice, practice, and practice!! Know how long your Impale lasts, and have your Shattered Earth already charged up before using it after Impale.

Previously, I mentioned that with Greater Quintessence of Movement Speed, Boots of Swiftness, Trinity Force, and Seismic Bastion, you will have over 510 move speed. Ok, why do you need that much movespeed?

Impale is a positioning tool that can win a teamfight. The more move speed you have, the faster you can bring back a victim with Impale for your greedy teammates, and it'll take longer for that victim to get away. And for many other reasons, move speed can be very convenient, but the main reason I get Boots of Swiftness and Greater Quintessence of Movement Speed is to synergize with his naturally fast movement speed and to make his ultimate even stronger.

Trinity Force not only increases your movement speed, but is a good burst dps item in general. And what's really scary about Trinity Force is it's Unique 25% Proc. To activate that proc more often, I get Guinsoo's Rageblade and Nashor's Tooth. As you can see, most of his abilities are AP, and by getting those two items, you'll do quite a lot of damage as well as constantly slowing enemies with Shattered Earth. With 8 stacks of Guinsoo's Rageblade, Nashor's Tooth, and Seismic Bastion, you'll have roughly about 1800 attack speed, which is crazy!! Oh, and did I mention that Nashor's Tooth also gives you mana regen and with mana regen runes you don't have to worry about your mana?

Hextech Gunblade is just overall great to have for a hybrid champion. Life steal and spell vamp will get your health back sooner than you realize. Don't forget to use the item!

Banshee's Veil for some extra magic resist. You'll already be doing a lot of damage by the time you're done getting Hextech Gunblade, and because you went 9/21/0 in masteries, you won't need many defensive items. Without Banshee's Veil, you have less magic resist than armor, so getting Banshee's Veil is the right choice.