League of Legends (LoL) Question: Help against Nasus

Two words, wave management. If at all possible freeze the wave outside of your tower and farm till you are lv 6 and avoid fighting him. Poke him with Q if you can safely do so, but make sure to contest him if he is trying to farm, especially those cannon minions. Once you get lv 6 and you've poked him to about half health fight him in the minion wave if possible then Ult him if he Ults or tries to run away.

I would also recommend getting Rylias first for the slow so you can stick to him better. I hope that helps you.

One tip I haven't seen mentioned is: ult him after he ults if possible. You steal more base stats that way.

Nasus is a pretty hard counter and it's worth to ban him if you'll only play Mordekaiser.

Other than banning, you have to zone him out of farm early on, when you can win the 1v1, and build Morellonomicon and/or Liandry's Torment, or even a Bramble Vest (but don't go full tank). Unless you completely deny him of farm (less than 200 stacks by min 20) and get a bunch of kills, he'll outscale you really hard and you'll die as soon as he Withers you. Make sure your team knows this and be ready to rotate to other lanes if they don't come to your aid. You can swap lanes with bot lane if dragon is not available, as you should fare better vs adc+supp than Nasus, or even swap with midlane if it's a better matchup for both (but you should know how to play mid lane).

In the end, don't expect to 1v1 him late game and try to play with your team.

If u want to be more tank, rush bramble, and tabi if the enemy team isn't full ap or something. If u want more AP morello is good. Early on E>Q>AA. Zone him off from stacks and ward.