Nasus just farms until he has 90 stacks then at lvl 6 he will destroy you. Theres a few exceptions to this rule but for the most part if you have 90 stack, you will win most of the fights. Don't be scared to take damage for some stacks. If you get too low remember that you have tp (at least if you thought ahead of time.) You are a passive laner for the first few levels. Level 6+ you can start looking for engages and possibly kill the enemy laner. After a few items you can 1v3. You will tank the most damage for sure and the enemy mid laner and jg will be like pests that you can get rid of easily.