For your skills I always max our Lay waste but always get defile second because every time you get a last hit at all with any of your skills you will get a small amount of mana back. your most used skill will be lay waste and defile. use lay waste as your harassing move constantly going back and forth spamming your Q. Defile is your team fight control move. what you should do is to wait for a team fight to start flash in press E and then activate Zhonya's hourglass. After that do not move unless and enemy team mate is running away with no health. If the team fight is still going stay there tell you die and play a suicide kart spamming your Q taking down all there health. Then after you die press your Ult. DO not press it when you are still alive in the middle of the team fight.
Your Ult can be stopped with a stun or a nock-up. If it is stopped it will go to CD and you will have wasted it. Use Wall of Pain to save team mates or slow escaping enemies.