For Nasus unless they have drawn a special jumpy team (ez, etc.) Ignite > flash. Almost always. Try it and you will see. Slow is perfect for getting on them and you should be building Phage/FM tbh.
"If the game lasts long enough and your able to finish your item set which would look like this:
Boots of mobility
Trinity force
Aegis of the Legion
Black Cleaver
Last Whisper
Infinity Edge
"
If the game has lasted that long and those are your items, you have lost. Sure you can 3-4 shot their carries but they will just destroy u. They have any health item your 1.4k crit won't do jack and they've already cut u up into small pieces and made the team fight 5v4. GG.
Nasus is designed to be a tanky dps. Build accordingly. Think Mord/Xin/Olaf.
Sallah