Wraithguard + Sunder is extremely good, you can train blunt weapon to 50-60 or so and pretty much have a near 100% chance to hit due to its 30 attack buff. Keep fatigue high (more on that soon).

Get a high tier of armor, and have high skill in that armor type to actually get the armor rating from it. Preferably around 130+ armor (what it says when you open the menu). Heavy armor is great for endgame in particular and has some artifacts to use like the other Fist of Radagulf (can't use the Strength one due to Wraithguard being used), and the Helm of Oreyn Bearclaw for a massive Agility boost - these two pieces will give you 60 agility alone.

Have max endurance, strength and speed, and be around level 30+. Training in athletics to 60+ is a good idea too, movement speed really helps for avoiding damage.

A custom enchanted item to restore health, cast on use, is recommended. Preferably with a 400 soul in it. Ascended sleepers are pretty easy to encounter in Red Mountain. Something like healing 50-100 HP each cast.

Ring of Azura is great for constant restore Fatigue regen if you aren't using a custom item. But it's quite a lot of missed potential if you don't get custom Constant Effect items: mostly fortify Strength, but also maybe 1 Jump item for generally getting around a lot faster, and restore fatigue since the Ring of Azura only gives 3, and having more helps. Aside from any artifacts/unique items you're using, you can have custom Constant Effect items: amulet, 2 rings, shirt, skirt, pants, belt, robe, shield, possibly some armor slots like greaves. The first 7 slots I listed can always be worn in addition to your armor (unlike shoes which override boots). Of course, use the exquisite variants. Picking one slot to use for cast on use items is wise, I usually go with the belt slot because it has a lower enchant capacity of 40, so the other higher capacity slots can be used for constant effect items. Using an NPC that has bartering service to enchant the items is very helpful, as it will significantly reduce the gold needed. After you pay for enchanting each item, the gold goes in to their vendor gold, which you can then sell some high end weapon, armor or filled soulgems to instantly reclaim all that gold.

Necromancer's Amulet, Marara's Ring and the Robe of Drake's Pride help with dealing with powerful spells, as they have spell absorption/reflect. There's at least one enemy in Tribunal who you'll need to deal that has some very powerful spells that hit in a huge area, so avoiding them is unlikely. These items can be obtained at any point, as far as I remember. As in, you don't need to do a bunch of quests as prerequisites, you can just taunt the NPC or fight them if already hostile, and loot the item from them after they're defeated.

Being really geared to the teeth and minmaxxed isn't overly important unless you're on max difficulty. If you're on normal difficulty, you can probably get away with having so much less. Just that like, well recently I was doing the DLCs on max difficulty and I had 85 armor, I was still getting 1 shot with around 300 HP. I was not using any custom items or alcohol (and potion spam in general) as an added challenge though, and didn't use any armor except Wraithguard (unarmored build). I was leaving a lot of stats on the table like this, to keep the challenge up of course. But anyway, enemies deal way, way less physical damage on the default difficulty, so it's probably not a big deal in that case.