1. Essences in Teganze
As stated before, they are quite.. rare. It isn't good build to farm it, will be slow and annoying to drink so many potions (you can't leech in this area). What helps great deal there is reanimates. But If i was going to farm either xis runes or traders chest upgrade (25 essences), I'd respec to summon barb. It is ridiculously easy with summoner, because summons take all the hits and they never die, take very little damage from those types of enemies.
2. Jday strategy
I've killed it multiple times this way in video, poison shrine and order I did. You don't technically need poison shrine as long as you gonna keep popping potions as soon as you get poisoned (it cancels poison). But you need to react fast, its impossible to dodge punisher while in melee.
Malthel is most dangerous with these deathstrikes, so it is important to move in time.
It is hard for me to give you more tips, because I really didn't have much problems with this quest, and tried it multiple times, always success.
3. Aidan's lament su crown
Good item, but nothing really spectacular. Charged strike from it is good, and makes it worth keeping, thats your astrogha kill right there, if you find this early.
Damage to undead is quite valuable, for example monsters in duncraig all souls are undead, so it works like EWD. That's part of the reason why assur rings are so good, they also have that mod. Demons are less common, can't think of uber that has lot of them, but it would be nice there too for sure.
Downside of this item is you gonna lose lot of life, and if you went full strength, you will lose lot of str from crafted creepy helm.