ur goal here is to farm as much as possible ( same as normal nasus ) and to rush essense reaver when you do that you have to look for spitpush or solo targets
crit nasus is great duelist with his prowler claw and u deal massive dmg when synergizing ur Q with ur e since it shread ur enemys armor
his biggest weakness is his health since you build crit and leathality u are super squishy so ur goal at teamfights is to prioritize adc and leap to them with ur prowler and let ur teamates tank the DMG is for any reason ur team does not have a tank u might need to e,ult,flash,prowler,q and pray
nasus is a great splitpusher for that reason u should prioritize getting towers in sidelanes so you put pressure on the enemy team cause they have to decide to either stop you from getting towers or stop ur teamates from taking baron or drake either way your team wins

there are some matchups or some particular situations that you might be aware of
for exaple if ur agaist a ranged top u might want to keep the waves freezed to focus on your farm and make the enemy always overextend if the want to farm cs , this also puts you in a situation that you are always safe from enemy ganks

since ur build is very low health you might want to make that ur advantage by taking lord dominics cause you in 97% of the situations will have less HP than ur opponent so you always make use of his effect , also it has a fantastic synergy with ur e because a max rank ur e shreads 45% of ur enemy's armor and lord dominics shreads 30% and in addition ur lethality items u destroy even the mightiest of the tanks

just be careful of enemy assasins i suggest banning always teemo :)

if ur enemy has heavy healing build mortal reminder\

if ur enemy have a lot of HP instead of armor tanks build Lord dominics and navori quickblade

and if ur enemy has a lot of cc or ap build mercurial scimitar

if it is a easy matchup pick ghost as ur summoning spell and look for agressive plays and if u kill them do not change ur playstyle and keep focusing on farm

since nasus is a very late game character i suggest picking teleport because u either want to wins early or stay to late game which means that u wins either way
u pick teleport so u can help ur team in teamfights so ur team wont be outnumbered and destroyed
personally (as an assasin main) u should look for cheesy flash Q to adc plays bc even if u dont kill the adc 1 tap it will be low enought tha he will have to retreat so you might win the teamight also
u should never engage first since u have teribble engage potentional