What you wanna try to do is bubble behind them or where you think they might go. Trust me I miss bubbles but I never miss them when it matters. My combo is bubble, damage/heal on bubbled person, steroid on my adc, and then ult. But if we know we are gonna kill them or he tells me to go all out I use my ult right after I bubble then the other things. Always open with a bubble. For bubbles its mostly knowing where people are gonna run to. Or if they avoid it by going out of there way you can punish them for it will aa's or a gank.
Here if you wanna see my nami status heres my lolking. [link]