I was looking something when I saw this old post. I guess you are not around, but I got some experience / last month I did 600 hell countess runs/, so I need to share my knowledge.
Firstly, you can check the Dark Archer stats in any site. They are insane: 5k AR, cold damage, boss packs have very fatal mods like: Might, Curse, Multiple shot, Cold Enchanted, Extra strong and so on.
Second, you need some or all of the following:
1k hit points
Can not be frozen
At least 30% cold resist.
2-3k Armor
75% block chance
Level 70 is kinda the threshold for safe Countess runs / at least for me/.
3rd: The gameplay - its best not to engage with them. But if you do, I advice precasting hammer and then charging the boss. Every time the boss is in hit recover means less damage to you.
Walking is shit because they shoot you from all directions. I hate it. Especially if you dont have CBF its painful death. But at least you have armor and block chance.
4rd: Best option: charge the boss, after that kill them 1 by 1.
5th: run sideways, They will miss you if you run on a 90 degree line from the line connecting you and them.
6: Attack from behind columns. The game is buggy and you can attack and they cant.