Realm Ranger Horde Dungeons Build Guide

Build Intro

The only attack based AoE skills in Orna are currently Arrowstorm (1 and 2) available from Hunter and Ranger specs respectively. The damage from Arrowstorm is actually divided among all enemies on the screen, so if your Arrowstorm was doing 100k damage to one enemy it would do approximately 20k damage to five enemies.
This build focuses on redlining, status immunity and high stats in order to solo horde mode themed dungeons or as a low tier party boss dungeon carry. For themed dungeons I don't believe it's particularly worth going for booster gear (orns/exp gear), as it's better to focus on survivability and completing the dungeons, where typically the rewards are Godforging or item hunting from event bosses.
I highly recommend using a decent amount of ward in order to use Gait of Gunnr for more offense. Something like +40k ward is probably necessary to use Gunnr for the harder themed dungeons, as there will always be some magic enemies who can deal heavy damage. If you have less than 35k it's safer to miss out Gunnr, stacking both your Def+Res with redlining and normal buffs to minimise incoming damage. Of course completing these dungeons in a party will always be safer as damage is not always focused on you.
Tactics are quite straightforward; buff up immediately with Golem's Fortitude + Barrier 2 + Ward of Ortanite (to tank any first floor hits), bump our ward absorption to 100% with Divine Bastion 2, finish our offensive buffs and increase ward turns with Fortify 3 or Ward of Ortanite. We then clear trash mobs with Arrowstorm 2, and have nuking single hit skills such as Purifying Strikes (I love this skill as it hits all physical immune enemies except Mighty Mimics, gives 1 ward turn and even recovers a small amount of ward) or Swordplay 3.

Video guide linked at the bottom of this guide!

Realm Ranger Gear

Weapon
I prefer to use a 1h weapon + shield to increase ward and gain stun immunity from Atlas shield. Going with your highest attack weapon is fine.  Using Arisen Teardrops adornments to gain some more ward is a solid choice if you are not geared enough to use whatever you want. Arrowstorm our main skill cannot crit and we want to redline anyway. Bident is good for extra foresight. 
(Arisen) Fey Badb Catha
(Arisen Badb Catha)
Arisen Ithra's Blade / Vulcan's Sword (Elementless skill may be good choice)
Arisen Bow of Tiamat/Typhon
(Arisen/Fey) Crowsong
Bident 
Finesse's Katana/Shotel (Good extra ward)
Realm Katana (Lower attack but good ward)

Armour
Initial focus will be high ward to increase our effective HP pool, before transitioning to high stats that we can boost further with redlining in Resurgence.
Old Nothren Crown
Fey Yeti Coat
Fey Yeti Horns
Apollyon Realm Gear
(Arisen) Fey Raven Boots - I like the boots only because we have limited mana for fey gear, helm and chest slots have better choices.
Arisen Ask Garb - The Temp All buff is actually quite regularly procced with hitting multiple enemies with Arrowstorm, providing boost to our damage and defense.
Tarnished King's Crown - Good for foresight and ward
Fey Yeti Horns
Adornments - Jewels of the End, Saturn's Eyes, Red Pinions - basically we want 2% ward jewels as a minimum in all our armour slots.

Off-hand
Atlas - Stun immunity and a decent chunk of ward is awesome especially as solo horde.
Arisen/Fallen Shield - High ward
Arisen Kerberos Shield - If you want to play a massive stat build instead of ward you can try this out (don't use Gait of Gunnr though)

Accessories
Without steadfast I really prefer to keep these slots simple and go for status immunity, especially as we are redlining at 1 HP. If playing in a party you can consider using presents/bag of treats for higher luck bonus and more drops if targeting items, or Band of Gods/Arisen Monster Tomes for exp/orns etc. You have to keep an eye out for dots though!
Briny Pendant
Ring of Annwn
Presents
Bag of Treats
Band of Gods
Arisen Monster Tome
Lugus Gauntlet - For ultimate carry build to get the triple attack up, better for party dungeons than solo horde.

​Pet
(Fey) Chimera for Berserk 1 buff is too good. +50% damage and helps us lower our HP into resurgence range makes this number one without much competition.

Realm Ranger Skills

Realm Ranger Stats

Bear in mind stats below are with 25 ascension level. Good initial targets for solo horde would be:
4.3k Attack
4.5k Defense
3.8k Resistance
40k Ward
​You can easily get away with less in parties.

Video guide covering all these concepts in detail whilst smashing out double Valley of the Gods solo!: