This is just a short guide to how I managed to scum my way through the Crucible despite, I admit, being a pretty average player. For the fourth stage, Waves 76-100, you're going to want to have a pair of strong possessed weapons to carry you through and a good idea of the kind of build from the skill trees you have.

WEAPONS AND ARMOUR
There are a couple of stats that you can focus on to make your possessed weapons into absolute beasts. Don't forget that when you're upgrading them, "feeding" a weapon another one with a certain stat will carry that stat over, allowing you to level up tactically.

SCYTHES
Critical Chance - combine it with a good talisman and you should easily be able to get >50%.
Health on crit - absolutely key. You can soak up hits from weak enemies without breaking a sweat when you combine it with a good critical chance, even on Apocalyptic NG+.
Critical damage - always nice if you can get it.
Wrath on crit - quite useful if you're planning to spam Exhume, Murder, and Aegis Guard.
You may also like some form of elemental damage to shock, freeze or immolate enemies. Alternatively, you could go for Execution, which has the added bonus of giving you a gory and satisfying ending to those pesky Wraiths.

HAMMER/AXE
Strength - this will bolster the damage of both your weapons.
Resistance - it's easy to neglect this stat and end up being one-shotted by the Deposed King. Make sure that doesn't happen and ideally use armour to boost both defense and strength.
Critical chance, health on crit, critical damage.

ARMOUR
In terms of the rest of your armour, strength, defense, and resistance are the core ones you need to go for. Put less priority on arcane, and ignore anything to do with gilt/experience as it's just a joke at this point. Abyssal armour is good, but if you use any, make sure you use every piece you have.
You can also use Blackroot's "Sticks and Stones" quest to improve that stats of your choice according to what stones you trade with him. Considering your Arcane stat may be suffering a little and the number of Mystic stonebites in the game, trading three of these in at once a few times is probably a good idea. The other key combination is Mystic, Power, and Power, which ups your critical chance by 2%. Finally don't forget Ostegoth will pay you with health boosts, wrath boosts, and skill points for his Relics.

SKILLS
I went for a full-out necromancer build and for many waves I could just hang out on the other side of the arena while my minions did the dirty work thanks to wrath on crit.

EXHUME
I used this on slow, heavy-hitter enemies as the exploding ghouls take off a lot of health and I was reasonably confident in my dodging abilities. They also make the Wicked K fight a complete cakewalk as using "Death's Allure" he won't even take a swing at you.

MURDER
If i was fighting numerous weaker enemies, or had lost some health from the previous wave, I would use this. It's not quite as strong as Exhume but is more applicable in the situations described above.

AEGIS GUARD
Certain enemies can do a huge amount of damage that is very difficult to avoid (Maelstroms, and I always struggled with Wraiths and Undead Stalkers). Aegis Guard is helpful in limiting the damage and allowing you to go to town with your secondary weapon without worrying overly about dodging. The Deathgrip is particularly useful for pulling yourself in to deal damage quickly before the effects wear off.

FRENZY
Wave 75 and Wave 100, Argul and the Avatar of Chaos, can tear through your health like a chainsaw through snow. Frenzy helps readdress the balance by doing pretty much the same to them. Despite the cost, the sheer killing power and long-range effectiveness make it a good bet towards the final waves, especially since you'll probably have plenty of Wrath potions that you didn't get round to using.

This build isn't going to let you one-shot the final boss, but I hope it'll give you some pointers as to what you want to be going for as you take on the crucible. :) If you think I'm an idiot and you have something immensely better and easier, let me know!