She can be very troublesome. The best way is to stay right on top of her and dash behind her when she starts to aim and fire. She cannot change directions very fast once she starts firing so you can just stay behind her. With this, you can beat her with almost any weapon as long as you can stay mobile and run/dodge when she goes for the AoE attacks, then run in close and continually dash behind her as she tries to shoot you.

Just keep at it. I thought she was much harder initially too but you learn how to deal with her.

Like the post above says, just dash behind her when she's aiming and keep swinging.

idk I'll try skulls on Eris but she didn't seem to be much trouble with Descura and Zoroapheth. usually I try to get as many Selene perks that give temporary armor so during inescapable situations just use Hex

she also quickly loses aggro when you hide behind pillars. I tend to use them as last resort

Try to take Hermes or Medea's projectile slowing Boons, they stack & also make a big difference vs. Eris (or any boss, really).

Athena's Divine Dash is a hard counter to Eris, that'll help you mitigate most of the damage. Dashing to/from the pillars between each of her phases works well, let her come to you, then bait her into the pillar.

For the big bombs and their AOE spread across the screen, I'd recommend Blinding Rush from Apollo, or any kind of Sprint/move speed boost helps.

Spam the Cast ability on Eris as much as possible, freeze/slow helps. Demeter's Freeze + Gust on Cast, plus projectile slow? Eris is toast.

My demo of using Demeter's Cast on Eris:
https://www.youtube.com/watch?v=TOgO1nWdWQk

If Eris is such a roadblock for you, I'd try going in and only focus on evading her attacks, don't even attack, just watch her and dodge and evade.
really just learn the moves.

she deals high damage, but all is pretty easy evadable, staying close helps.
it all has a distinct animation, so you know if it's a dash, the shotgun or a snipe

over time Eris went in my internal ranking from pretty high, to almost the easiest.

If you're not under time pressure, you can cheese the Eric fight with nearly any weapon by hugging the two pillars. Many of the weapons will go through them and let you damage her while you are invulnerable, including your cast.

The faster strategies involve getting and staying behind her, but the pillars offer a much safer alternative if you're having a difficult time.

You know, I've seen topics like this about basically every surface boss. "If only I beat the cyclops the rest is breese", "after Eris Prometheus and Typhon feel easy", "Prometheus is the worst boss ever", "Typhon is ♥♥♥♥♥♥♥♥".
With Eris... Pillars are a great answer, you want to keep her sniper shot away from there but otherwise hug the pillars. Mental Block/Divine Dash from Athena really help in this fight, great in general but if you are feeling risky you can bring her down to the buff zone, then when she starts shooting try to deflect a couple of shots back to her: if it is the shotgun, you may even bring her down to the next buff threshold with a well-placed Mental Block.
She seems like she is dangerous from afar, but it is mid-range which is the worst, if you stay far enough you may get out of her shotgun/full auto range with a single dash(hard to do with most weapons, but a cast build can stay out of her range completely)
Do not rely on Frinos to block anything other than sniper shots though: he gets pushed back with each shot