TL;DR:

Level with jung's PS mines: https://www.youtube.com/watch?v=Vs12_bjELNA

Barebones POB for swap: https://pobb.in/9lRwftUEm3EL

#Getting Ephemeral Edge

1. Farm Wolf's Shadow in Dark Forest
2. Turn it in on a level 2 character
3. Ancient orb it for EE (8 ancients on average)

#Getting Clusters

Atlas tree: https://poeplanner.com/a/Oh7

You just need those nodes, you don't need this specific atlas tree.

Just do white tier 3 Strands, you don't need them to be high level.

The clusters you need are "Attack Damage while Holding a Shield." Ideally you would go straight into 8 passive, but 9 and 10 passive are definitely usable while you farm for the 8 passive ones since they allow you to get Veterand Defenders and sockets for more Cobalt Jewels.

Additionally, you can fit a Projectile Damage cluster with Eye to Eye and Repeater for more projectile damage and attack speed.

This will probably take a while and a bunch of maps, so strap down for it. One thing you can do to make this go faster is to add Delirium Scarab of Paranoia, which will make it drop a lot more clusters per reward wave. I didn't have these when I farmed mine so it took pretty long, but it's pretty chill farm overall. I wouldn't farm more than 1 cluster before going into LS, as it feels much better to do this with LS than PS.

#Rolling Clusters

The most important thing to get is Veteran Defender, it gives you ES, attributes and resists.

For the starting clusters, I would roll only Veteran Defender and anything else helpful you might get. (harvest reforge defense, chaos spam, alt spam, etc).

If you want to go for 3 notable clusters, what you do is Harvest reforge Speed and exalt bases with 2 notables for the third one.

Apart from Veteran Defender, the best notables are Feed the Fury and Martial Prowess. That's what your ideal clusters would have on them. Fuel the Fight is also good because it solves mana leech.

#Acquiring Bases

The bases you need are the following: ilvl 84 Warlock Boots, ilvl 84 Warlock Gloves, ilvl 84 Lich's Circlet, ilvl 86 Necrotic Armour and ilvl 86 Titanium Spirit Shield.

You should add these to your filter and pick up any that you find. If they are lower ilvl than you need, which a lot of them are, you can use the recombinator to get them to the required level. The ilvl of a recombinated item is equal to the average of the two input items + 2, but it is clamped to the highest ilvl of the two items (you can't put in two ilvl 84 items to get an 86 one out).

You can get feeder ilvl 86+ items in a bunch of ways: T16 Oba's Cursed Trove, T16 Coward's Trial, shipping, Gwennen, Rog (you need to be level 90 for the last 3. I'm positive this is the case for Rog and Gwennen, but I'm not 100% sure for shipping).

Once you are level 86 (ideally 90 I think), you can start doing shipments to get your starting bases. You can send Crimson Iron bars to Ribben Fell for mixed bases, this has a chance of getting you the items you need for the build. I haven't done this and instead went to T16 maps and dropped them that way.

#Crafting Your Bases

First of all, prepare to do a bunch of Expedition because you'll need a lot of Alterations and that's probably the best way to get them.

After you farm your alts, you need to start alt spamming bases for T1 ES prefixes (make sure your bases have the correct ilvl so you can actually roll T1). Afterwards, you just put 1 prefix items together in the recombinator to make 2 prefix items and slam 2 prefix items together to make 3 prefix items. There are guides for more advanced thechniques with exclusive mods and NNN blocking and stuff like that, but those require divines and metacrafts. The 1+1 -> 2+2 method works fine for 3 prefix whatever suffix items.

For the rings, you just pick up some Two-Stone Rings and do Harvest reforge Defence until you get ES and evasion prefixes (you need evasion too because of the ES/EV mastery) and any useful suffixes (mostly resistances to get to cap).

For the starting amulet, you just do the same thing as with the rings and try to get double ES prefixes (flat and %) and any good suffixes you land on. Agate Amulet is probably the best here since you need a bunch of strength. You don't ever need Voice of the Storm, the builds functions perfectly well without it, it's just a nice upgrade to shoot for.

#Crafting Cobalt Jewels

Harvest reforge Defence until you get maximum ES for starter jewels. Afterwards, the same thing but you just shoot for more useful affixes (attack speed is really really good to have, int is nice, attributes are nice, resistances are nice, etc) and exalt good jewels.

#Tattoos

While playing the build, make sure you send 200k (no dust, I think) shipments to Ngakanu for tattoos. You want 10 Tasalio Tideshifter tattoos for stun immunity. Before you get those, use Brine King pantheon. You can also pick up the Perfectionist cluster on the tree for 60% stun avoidance so you can be immune with just 4 tattoos while you try to get the rest.

Hinekora Warrior tattoos are also good. They don't add an insane amount of ES, but every little bit feels good to have.

#Swapping

For the swap, you're just shooting for 10k ES and that's more than enough to start farming with the build. The most important (probably mandatory) pieces you need for the swap are:

3T1 ES helm, 3T1 ES chest, 1 cluster with Veteran Defender and a bunch of %ES Cobalt Jewels.

For the rest of the items, just try to get anything usable to get you to 10k ES and res cap. You can use Dense Fossils to craft your gloves, shield and boots. You're just looking to get enough ES on them to reach 10k. For belt, rings and amulet, you can usually get some okay ones from Rog by this point, if you don't have the harvest juice to roll them. You can even chaos spam some of them to get usable items.

Also, get Vaal Lightning Strike as soon as possible, if you don't have it leveled already. It's a huge help on single target, especially on lower gear levels.

Ascendancy

The only mandatory nodes, in my opinion, are Escape Artist and Soul Drinker. I personally wouldn't drop Spellbreaker even with Ghost Reaver because the Suppression it provides makes gearing a lot easier and 10% additional suppressed damage is really nice against big spell hits. I'm a big fan of One Step Ahead, especially for mapping, but Polymath is a good choice for really good damage.

Another way you can go with the build is to pick up Swift Killer and get the endurance generation mod on the body armour. With the increased charge duration from Swift Killer, you can always be at max endurance charges that way for extra tankiness. The +2 frenzies are also a bit more damage.

Upgrade Path

Once you're swapped into LS, you can continue farming to get better and better gear. The first thing I would farm is probably the clusters to get into the 3 clusters version. This will get you a lot of ES, attributes and res, which will make gearing a lot easier. Additionally, it will allow you get more Cobalt Jewels in, which will give you more ES, which will give you both more survivability and more damage.

After that, you would probably want to continue crafting your gloves, boots and shield to have 3T1 ES prefixes.

Other good things you can farm are Crusader rings and (Crystal) belt. The influence will allow you to get %ES mods on them too.

Harvest enchanting your sword with attack speed per quality is a pretty nice upgrade too. The build really wants attack speed to feel good, especially when you have Multistrike.

Resolute Technique EE is a nice luxury upgrade, which will allow you to drop the accuracy nodes on the tree. You can also try to get an accuracy roll on your helmet while recombinating.

After that, you would start farming for stuff like ES Light of Meaning, Shavronne's Revelation, 3p2s rares, 3/4 stat jewels and other high end upgrades like that. You can get a lot of mileage out of this build before you have nothing else to upgrade but it feels good from start to finish.

Versions

There are a lot of versions of this build, so you can take it in any direction you like. In no particular order, some of them are:

Block with Aegis

Block with Shaper Shield (more ssf oriented version of above)

Sword Clusters version for more damage and rage generation

Triple ele flask with Balbala

Max res stacking with Melding

Splitting Steel with Nimis

Quadruple Curse with Anathema

Int Stacking

Wicked Ward version

And probably a lot more I'm not familiar with, so don't be afraid to tweak the build as you see fit for your own needs and playstyle.

Conclusion

Overall, I think this build is one of the best you can play on SSF, in regards to both damage/tankiness and easy of setup and play. It feels really good for both clear and single target, it's extremely flexible and extremely tanky. I'm already at level 99.5 in SSFHC with it, I'm far from done with upgrades and I've been running alch and go T16s from level 94 without any close calls whatsoever, so if you're looking to start (HC)SSF in 3.25v2, this is definitely one of the best builds for it.

Let me know if there's anything I'm missing, anything I'm wrong about or anything you would like to see added and I'll do my best to answer any questions you might have.

Best of luck!