Doom Amulet can, in fact, be quite challenging. Keep in mind that if the shards are destroyed the Doom Amulet will bring them back after a time.
I'd suggest that you focus on the amulet first and then the shards since the shards will do nothing after the amulet itself is destroyed.
If your original class is warrior class, focus on to invest a little bit in STR.
For mage class, INT, and for rogue class DEX.
One good investment would be, for example: 100 STR, 100 END, 100 LUCK and the rest on WIS.
Luck is very good to give yourself a greater chance for avoiding your foe's attacks.
Make sure you equip as high level accessories and weapon as possible before the fight, it will make the battle a little easier.
Remember, you have Artix and Nythera with you so use Artix powerful multiple strike ability and Nythera's debuff ability on Doom Amulet.
Heal them when their health is at, for example, 70-80%. It's the best time to do it early. Go on the defensive as much as possible because this battle, especially the final battle with Doom Amulet, requires patience and endurance.
Watch out for attack 1, 4, 5 and 6. Then you need to be extra careful. In this link you can look at what kind of attacks it's making and what effects they give and develop your strategies.
When it performs attack one it would be good if you shield yourself as well. Don't attack until it's defenses has run out, then you can blind it. After the blind effect has stopped you can then weaken it's boost power as often as possible.
When your allies and you have taken heavy damage it is time to heal, debuff the Doom Amulet as much as possible and boost yourself up and use your strongest attacks.
The Riftwalker (or atealan armors) is good but so is Evolved Pumpkin Lord Armor, Master Soulweaver and enTropy Armor.
I hope this helps you! Good luck!
EDIT: Get the Not-So-Tiny-Bubbles Trinket from the quest Questions! It is a really good trinket for tough battles.
EDIT 2: If you're up for it you can also try to farm for the Unhallowed DeathKnight Amulet lvl 70 version. It has some powerful buffs.
< Message edited by UltimateDoomKnight -- 7/22/2017 12:47:12 >