The Nex boss is available in 2 wilderness locations, one in multi combat and another one single combat. Both of them have the same drop table, and same attacks (final nex phase). Keep in mind that the Nex boss is extremely difficult to challenge and it's levels (excluding hitpoints) raise based on the total number of attackers. There are semi-direct teleports to the boss via the Teleport Wizard. (::nexmulti & ::nex)
The easiest way to fight Nex is to camp Ranged with the best crossbow you can get your hands on. Keep in mind that while Armadyl IS technically better than Karil's or Ancient d'hide, it's far less cost effective and you're better off saving to upgrade to void and skipping the Armadyl armour altogether. The most important stats are your Ranged attack, Ranged strength, Magic defence, Crush defence, and Prayer bonus.