First of all - the old Etched Sigil has two major flaws when it comes to Frozen Orb. First being that the damage buff you receive is weak. While a x2.5 multiploer plus autocasts is good on paper, you have to compare that to the buff you’d get by using Triumvirate. Plus autocasted Orbs can’t deal area damage, and that’s sad for Frozen Orb.
Anyway… as you wish. You won’t get very far, but it’s doable.
First things first - Deathwish. Forget that thing, it’s a major pain in the a$$ because of that 1 second delay they added to counter Bazooka. When it comes to Channeling it’s possible with Tal Rasha, but for Arcane Orb let’s quickly forget that set, same as a LoD or LoN based build. Thing is, they will be slow like a snail, since… no Teleport unlock unless we use Aetherwalker and that’s a NO-GO for Arcane Orb with legacy Etched Sigil, that’s gimping our damage way too hard. DMO is an option, yet I would still go for Firebird. As weird as it sounds, playing a Cold build with that set.
Reason is free Teleport and you want to use Disintegrate anyways - we will put that to use. That way you can, at least, use two of the Arcane Orb items plus wearing legacy Etched Sigil. Now here’s the real reason as to why Deathwish is not recommended. You can simply look at the figures. Triumvirate is a source, and since a legacy Etched Sigil blocks the offhand slot, we would have to cube the source. We won’t do so, but the fact remains anyways…
So, for the weapon slot you may choose between three, being Deathwish, Wizardspike and Unstable Scepter. Kindly look at the figures, it’s clear that Unstable Scepter will outmatch Deathwish and it’s also stronger than Wizardspike. So Unstable Scepter will be the go-to, provided you have a useful one. That leaves the cube slot… Damage wise it could be Deathwish or Wizardspike, but a 1 second channel delay until the buff from Deathwish kicks in make the sword pale in comparison. Wizardspike just grants that damage (even a bit more) upfront, no strings attached. And you get an occaisonal autocast Frozen Orb on top. Bye Bye Deathwish.
We could also go for Triumvirate here, the damage buff is really good, but you have to add a signature skill into the build to keep that one up, in addition to the Disintegrate, Arcane Orb and Teleport skills we already use. We don’t want to play Tal Rasha, no? t’s disturbing, so no Triumvirate. So, now the gearing:
Firebird: HELM (Topaz), CHEST, SHOULDER, HANDS, LEGS
Captain Crimson: BELT+BOOTS
AMU: Squirts (Fire%)
RING 1: SoJ (Fire%). CoE )damage) or Karini (defensive)
BRACER: Ashnagarr (Fire%)
WEAPON: Unstable Scepter
OFFHAND: Legacy Etched Sigil
CUBE: Wizardspike, Pride’s Fall/Aquila Cuirass, RoRG
LEG GEMS: Taeguk, Bane of the Trapped and either Zei’s or Invigorating Gemstone
Skills:
Teleport (Calamity or Safe Passage)
Arcane Orb (FrozenOrb)
Storm Armor (Power of the Storm)
Magic Weapon (Deflection)
Familiar (Arkanot)
Disintegrate (Chaos Nexus)
Passives:
Audacity, Power Hungry, Galvanizing Ward… and whatever you need (I took Evocation)
I just slapped the items I had in my stash together. Mostly primals with 120-130 augs, so to make up for that I removed all paragons from INT and the Topaz gems from the chest and pants.
Playstyle is laid-back. Just button down Disintegrate and then hop around by casting Teleport. You can also throw in manual Arcane Orbs on Elites. Lazily played some GR’s with that… T16 is a no-brainer. As is up to GR90… I cleared a couple of them at around 6-7 monutes (a bit slow, but I did not use any of the pylons to make the rifts a bit more comparable)… I can tell it works, but not too well. I’d rather not recommend such a build but at the very least every single RG dropped a gift 
