so what would you suggest to improve this build?

Not enough mana and not enough speed. Speed is the key for skarner.
More speed = running away faster (you'll need that because you die in a blink).
AND chasing faster. When you attack you want to be able to chase your opponent to death.
When you run out of mana you'll see you're pretty useless and can't really touch your opponets without too much risks.