Your Nasus will run OOM all the time since you're not having either runes OR items for mana. Please look into that as you're also implying the heavy use of mana due to ranking Spirit Fire right after Siphoning Strike. (I'm sorry but your Tiamat gives you basically nothing in terms of MRegen)

Also, your playstyle looks to be like a physical carry, of which there are both better and tons more. Because you have no survivability, life will get tough when you're supposed to be fighting people. You need the Q farm to do damage, and I've hit 1k crits on people before just by farming well enough and building tank. Your AD doesn't help you stack any other abilities, and your Q takes care of things so well anyway, that there's no need to focus.