I couldn't remember if you could unseal the door to the Hall of Echoes and complete that quest before actually entering, but I had a recent save, so tried that on the way back from the Gremory chest. It turns out you get the experience as you enter, so I decided to check out the Hall of Echoes, even though I had a couple minor things left to do.
In principle the changes to the memory eaters looked good, but the difficulty spike seemed insane. A Fire Sphere could only do about 5-10% damage, and unless they were at the far end of range, there was no way to avoid their seeking attacks. I started doing ok once by taking out the first couple nests and then circling around the area high up, but by the time I took out the first memory eater there were half a dozen more, making it impossible to survive.
After dying several times, I exited to the main menu rather than load a save, then decided to give it one more shot before I went to bed, so loaded the auto-save. The game seemingly changed the difficulty level (the setting remained on Nightmare) as this time the memory eaters were easy to defeat, attacks were relatively easy to avoid and the nests seemed slower spitting out opponents. I kept going when I was done, and the arena was also fairly easy.
The next day I reloaded the save at the Gremory chest, and double checked the difficulty setting, not wanting to make it through the Hall of Echoes only because the game decided to make it easier.
...
I returned to the battle tower and retrieved a couple pieces of dragon equipment from the chest, that had a shield skill bonus (and I put my 3 spare points into that skill), as well as the +40% defence bonus leggings (which I didn't end up needing). Earlier I removed all dragon equipment except for the Bone Dragon Cuirass (+40% hit points) and Bone Dragon Leggings (+40% offence) once I could max Firebreath, Fire Sphere, Dragon Spirit and Dragon Burst without the need for equipment spell bonuses.
I only had about 10 each of healing and rejuvenation potions, so farmed for enough ingredients to make a stack of each (while wearing my +3 Master Herbalist earrings, of course), as well as gold ore to add some enchantments to my new jewellery. The earrings had a level 4 Healing Aura enchantment, so I left it and changed the other to Static Charge; for the bracelet I removed all 3 enchantments and added Static Charge, Magic Damage Aura and a Healing Aura (my second malachite gem used).
Returning to the Hall of Echoes, the memory eaters were tough, but not nearly as much as in my first attempt (the Dragon Shield spell also helped a bit). The killers could be killed with 2 (or sometimes 3) Fire Spheres, while the generals needed 3-5, depending on how much they healed; attacks at mid range (that I could see coming) could usually be avoided. Two memory eaters at a time were not too dangerous, especially if they were close together. Three increased the difficulty, and 4 started to make things quite dangerous.
Unlike D2:ED, it was ineffective to circle around nests while using target lock (and having a flaky target lock in DKS didn't help either). I found it best if there was a memory eater in the distance when going after a nest to try and sit on the opposite side while using Firebreath (to hopefully block any seeking attacks), then hightail it out of there to avoid taking too much damage from the memory eater if it got close (fly, heal, then turn and use Fire Sphere).
The arena was fairly challenging. In the previous play session (when the game made things easy) I didn't even summon my creature, and think I only needed 1 potion. This time combat became much more hectic, with more and tougher summoned minions, which required more potions and strategic running away tactics in a couple places. My creature even died, which hasn't happened very much when it started off a fight at full health.
I'm assuming the difficulty level when I first entered the Hall of Echoes was a glitch, and the second was how it was meant to be. I don't see how it would be possible to take out all the nests with Fire Sphere doing so little damage to the memory eaters (the nests didn't seem to be particularly resistant to Firebreath). I also had a problem the first time I encountered the large group below the Grand Knight teleporter shrine in the fjords, where opponents barely took any damage, especially at long range (the next day I loaded the same save and tried again, and they took normal damage).
In Ygerna's memory, my first attempt lasted about a second, with the Divine Rush Attacking me as I jumped backwards, and me dying before I could heal. The next try I jumped back slightly sooner after the conversation ended. I still got hit with a Rush Attack, but didn't take as much damage, and jumped off to the side and got around the corner before using a healing potion. Lucian was quite dangerous at medium range; at long rang there was time to react, and at short range he didn't use Rush Attack, so I could usually get off some Splitting Arrows, before running away (often needing to heal). My creature and a couple explosive arrows took care of most of the weaker opponents, with a poison and normal arrow finishing off a couple that survived. When I defeated Lucian, Zandalor was closing in, so I had to run around the room again, to give me time to grab the loot bag and pick up the skill book [ED: by the stairs from the starting corridor]. My creature apparently diverted Zandalor's attention on the opposite side of the room as the entrance, so once I had the loot I got first shot in, and we traded a couple arrows and spells until I finished him off. I tried circling around when his health got low, to see if I could use a close range splitting arrow to finish him off and have time to grab any loot, but the cutscene started immediately.
...
At the end of the Hall of Echoes, I had 98 unused stat points (base stat values the same as they were at level 15), and 21 unused skill points.
equipment with without
hit points 942 306
mana 639 222
vitality 53 14
spirit 22 2
strength 21 21
dexterity 51 36
intelligence 7 7
damage 281-425
magic damage 114-133
melee resistance 22.1 10.4
ranged resistance 38.5 16.6
magic resistance 37.2 3.7
conditioned body 21 21
heightened reflexes 74 36
indomitable will 7 7
Ranger Strength - 10
Explosive Arrows - 6+8
Splitting Arrows - 7+3 : these 3 I should probably max soon
Poison Arrows - 1
Stun Arrows - 1 : secondary skills
Death Blow - 1
Evade - 1
Healing - 1
Regenerate - 1 : tertiary skills
Lockpick - 5
Ranger Stealth - 5
Mana Efficiency - 6
Mindread - 15
Before the battle tower I had a couple points in Poison and Stun Arrows, as well as equipment bonuses (recently replaced some +2 Poison Arrows earrings). Both still come in handy, but mostly just as a way to pause and select a specific target, then come out of pause attacking (a normal ranged attack doesn't unpause the game).
