Singularity is by far the most damaging rune here. If you take Razeths out, you are going to suffer a big damage loss. But it’s a good thing to not have max esssence when you run around cause the shoulders prepare you for that by giving 50% dmg reduction. I used to have both Razeths and aquila so I could have that 50% dmg reduction all the time. The one completes the other.
Don’t really remember my exact skills passives and items cause I salvaged most of them due to lack of stash space but it was something like this.
Leoric crown with amethyst gem
Razeths shoulders
Aquila for torso and the chest requiem cereplate something like that that helps refill your essence was in the cube
Ice climbers (boots) - stone gauntlets (gloves) for that massive toughness. Remember you need ice climbers, cause otherwise stone gauntlets make you unable to move and attack. The boots negate this debuff. Or if you don’t thing you need that you can go with tasker and theo and steaurts greaves + bloodrush.
Pants were either the golemskin or the swamp land with elemental percentages (can’t remember which but both work)
Jewelry: haunted visions, nailuj’s evol (julian love backwards lol), krisbin sentence and the fourth can be either unity for toughness (give also 1 to your follower paired with immortality token) or convention of elements for damage.
Belt was dayntees binding. You also need a curse. Aura of frailty is the easiest to play with since you don’t need to do anything other than to be close to enemies.
Weapon was scythe of the cicle and phylactery doesn’t really matter as long as it has good stats like skeletal mage dmg, critical hit chance, etc…and in the cube was reilena shadowhook
As for important skills and passives, I remember dark reaping was amazing to refill your essence, and the other passive that increases the duration of skeletal mages, blood mages for skeletal archers that helps even more to have them up for longer, or the cold rune paired with lost time phylactery for great movement speed.
The more you play, the better you become and you can remove defensive gear and replace them with offensive as you progress and improve. Keep in mind this is not the easiest build in the game, you still have to watch for a few things but this defensive - no cooldown setup that I described is actually quite easy once you get the hang of it and very good for speeds and farming.
Edit: You might wanna take a look at this.
https://www.youtube.com/watch?v=xIIo2W26WVc&list=PLOWnTa0Nars73vmbmRKwWsrnOU2r1V4P4&index=10&ab_channel=Mnem
His is a bit different than mine, but from what i saw it works perfectly. Look at his skill bar, 2 of the 6 skills are auras and 1 is simulacrum that you only need to cast once, so basically it’s GR99+ with 3 buttons. And as i said, nothing is written in stone. The more you improve, the more changes you can make but some things like the simulacrum, or the singularity or the krysbin sentence are almost mandatory for the huge damage they provide.