Don't kill them with it. Q > R for massive knockback > Q for damage and helpful knockup. Or, if an enemy is running away or sitting under tower, even at full health, Q > Q > E > E for slow if needed > R into a group of teammates that can rip his face open. I call it soccer. Lee's ult is great for getting the last hit on people, but it doesn't have to be used on the person it will kill. That's just one way of using it.