Basic attacks reduce the cooldown of Crystal Slash. Use them as often as possible for maximum damage.
Capturing Crystal Spires before attempting neutral objectives or team fights around them improves Skarner's performance in said fights dramatically.
Impale is extremely powerful when you use it to position an enemy so your allies can attack them.
Skarner can't harm you very much outside of short range, so stay away from him!
Skarner's Movement Speed is increased as long as his shield, Crystalline Exoskeleton, persists. Attack him to lower his speed.
Skarner is especially dangerous when inside Crystal Spire zones his team controls. Try to capture them ahead of time if a fight may break out around them.