"Nex" is one of the most famous bosses in Runescape. In DreamScape 317 he isn't that hard to kill.
The best way to kill Nex is with a Brutal Whip. If you use your special attack with the Brutal Whip, you will one hit Nex every time. If you do not have a Brutal Whip, it is recommended to get an Ice Katana, Glock, or a Assault Rifle, and pray Soulsplit. (Level 92 prayer required).
Since the Nex is a boss, it is included in the Soul System, converting all common and uncommon drops into souls. The Nex, along with all other NPC's in Dreamscape, drop Key Fragments and Treasure Chests.