Nomad can be fought as many times as you'd like per week, however only the first kill will give Nomad's rewards.

Strategy

Phase 1:

You will be getting hit for a lot of typeless damage that you cannot block (1,000 damage~). Don't use soulsplit, it causes Nomad to heal for a lot every couple of seconds. The first special used here is where Nomad teleports you to the middle and Gielinor will smash his hand on a location. Simply surge or escape your way out of this. Protect from Melee prayers will reduce the damage of the attack. There will also be black spots that appear on the ground. Fireballs will travel towards these spots and hit that area for 2,000 magic damage.

Phase 2:

Nomad gains a new special attack, in addition to the ones he had in Phase 1. This special attack is where Nomad will teleport you to one of the ends and use an attack. This attack can deal 7,500 or 9,000 damage depending on a choice you made during the Dishonour Among Thieves quest. You can use Resonance to block the attack, or use Debilitate to halve the damage of the attack. Alternatively, you can run behind a pillar after he teleports you, or use Barricade. However, barricading is not recommended as you waste all of your adrenaline.

When Nomad teleports you for an special attack, you have about a 2-3 second window to do some more damage. You can get about 3 more abilities in before you need to activate your Resonance or Debilitate. You could stretch it even further, as the damage dealt from soul blast goes off only when the animation actually hits your character. Remember that Gielinor's attacks still go on in the background. Simply run out of Gielinor's attacks.

Phase 3:

In addition to his specials from phase 1 and 2, during phase 3, Nomad gains yet another special. This time at half health, Nomad will form a clone; there will be a Nomad in the middle of the bridge and a Nomad on the side of the bridge. When the clone is out, Nomad will not use his Phase 1 and 2 specials, but Gielinor will still attack. If you kill the real one, the phases will continue

Phase 4:

On this phase, Nomad does not use any of his specials. Instead, he will attack rapidly with Melee. However, He will turn off you prayer. Remember to use Devotion or Debilitate abilities if necessary. It is also important to use your bleeds in this phase.
Gielinor will continue attacking at this phase as well.

Things to remember

When a phase is transitioning, you can still get hit by Gielinor's if you killed Nomad when Gielinor launched an attack.
Use the Sunshine or Berserk abilities whenever you can. Move Nomad whenever you use Bleeds on him to get 2x/3x damage.
Protect Melee all phases
You regain all your health after each phase
Stuns work on Nomad
Keep applying Dreadnips!
Throughout all phases, Gielinor in the background will be launching special attacks of his own, so be wary of that.