This guide is going to be straight to the point. Nex is practically AFK outside of a few mechanics that could kill you if you don't pay attention to them.
Here's what kind of gear I highly recommend before attempting Nex: (runes for blood barrage)
Preparation for Nex
Before you head over to Nex, you should make sure to get a bonfire boost using Evil Bark, obtained from players or the Evil Tree. This bonfire boost will last for 2 hours.
If you want to stay at nex for the entire two hours, I recommend bringing less sara brew flasks, a second holy overload, and more super restores.
If you have the Prayer Betrayer perk, then you may not need many super restore flasks depending on your prayer bonus.
If you don't have a max cape/comp cape then you'll need the frozen key, teleport to GWD, then run to the frozen door directly South of the teleport.
An alternative method is with the Max/Comp Cape teleport to the Max Guild. You have to attune your portal to Nex. This will send you straight to the Nex lobby.
The last way to get to Nex is through the ;;bp command for diamond donators+ (no picture:most diamond know this already)
If you have the DPS to soul split and not need food most of the fight, I'd use Soulsplit through the fight. If you don't feel competent enough to fight Nex without protection prayers, then for P1 you'll protect magic. If you get poisoned during this phase, take a sip of your super anti-poison. P1 is very simple. There's no deadly mechanics that'll stack you out. Whenever Nex calls for Fumus, then kill Fumus and move to the next corner. This is where I stand as soon as I enter the Nex room so that I can splash both Nex and Fumus, so that whenever Nex calls for the help of Fumus your attack won't be voided, and will still hit Fumus. This is just an optimization I found that helps speed up kills.
Prayer: Ranged/Soulsplit. Phase 2 has this highest chance to die, as Nex has an attack that will hit 1,000 damage easily. Nex will say "Fear the shadow!" and you'll see a black circle under you. To avoid this attack simply walk at least one tile away from the circle. Nex will call for Umbra when low enough health, kill umbra then immediately run to the next corner. Nex's first mechanic right after Umbra is killed will be a siphon. Any incoming damage to Nex will heal her.
Prayers: Magic/Soulsplit. The only mechanic that you'll need to know for this phase is when your player turns fully red and Nex says "I demand a blood sacrifice!". All you have to do to avoid this attack is to click at least half way across the room. You don't actually have to run over there, just click that way. If you miss your window to avoid this attack, you get hit with a prayer draining attack that's pretty accurate. Kill Cruor when Nex calls for him then move onto the next corner.
Prayers: Magic/Soulsplit. This is the second most dangerous phase because Nex has two attacks that will drop your prayer, and one of them hits 700's every time. You'll know when the attacks are coming because Nex will say "Contain this!" and the attack will hit a few seconds after. This attack will either send 4 spikes around Nex and disable all prayers if you're hit by it, or trap you in an ice prison. If you have a shield capable of the freedom ability, then you're able to escape this prison by yourself, otherwise you have to have someone else click on one of the middle ice spikes to release you and avoid all damage. These are what the attacks look like.
Prayers: Magic/Soulsplit. This phase doesn't have any mechanics other than the death mechanic. Whenever Nex dies, she will turn on the wrath prayer and then damage you if you're nearby.