It would depend on which NMs. I have used high Eva set ups, as well as M.Def Set ups vs NMs who cast high lv spells. i.e. using a +100 resist + gear resist + M.Def atma + 3rd DD type atma. Some NMs use status effects which can highly hinder or kill you, so protection against those is also viable option. I have mnk, nin, thf all with full AF+2 as well as other end game gear, and kannagi Nin. I can not say there is truly only one way to tank or atma set to use.
The best bet is do your research and be prepared. It is quite easy to kill near all NMs/Boss's in game solo or duo with a good healer when you are prepared to do so.
If you have a specific question to a specific NM, I would be glad to share how I went about killing it, that is if it is one that I have killed (*^.^*)