After Trinity Force, if the other team has a lot of AP, then build Force of Nature instead of Phantom Dancer. This is a really expensive build, so once you get to Nashor's, then buy according to the enemy team. I they have high ap, FoT. If they have high defense, then buy Black Cleaver. The idea here is to be able to move around the map quickly for ganking, be able to get away, clear the jungle quickly (use smite if you want, i don't prefer it since it is useless late game), and the most fun part: activate ghost, then exoskeleton, then ult someone, and drag them far far away. This build is clearly ad and critical chance. Rather than rely on your abilities with an ap build, focus on auto-attacking.
Early game, harass with Fracture, and keep the enemy team out of lane. I don't jungle with Skarner until at least level 4 because quite frankly, he can't do it early game. Never use Exoskeleton unless you are chasing down a champion. It sucks for them to survive due to flash, ghost or stuns etc. Also, just because he has really low cooldowns, don't spam abilities unless you have blue buff. You will run out of mana and have less impact. Wow writing 5000 characters in this guide is really tough, that's a lot of letters. Let's see, its important to spam Q while chasing an enemy to constantly and endlessly reapply the slow of the ability. otherwise, the damage output is so minimal that you should just save the mana, unless you need a heal. Then use fracture and follow up with Q to hit all of the enemies and heal a lot. Crystalline exoskeleton again should be saved for ganking, chasing and fleeing. I always target any enemy champions with crowd control, especially stun, with impale. This way, obviously, they cant stun you or you allies. If none of them have cc, then I go for any ranged, ad carries, or burst mages. Some people build cooldown reduction and attack speed, but it isnt really necessary since skarner's cool downs are not that long anyway. What are we supposed to type about in 5000 letters? That's ridiculous. I'm at 2000, and there is nothing else to cover. Skarner is a scorpion. He should have some kind of poison ability or something scorpion-like. Impale should animate to actually impale people. Okay I give up getting 5000 letters.