Eula, despite being a Cryo character, will be dealing a majority of her damage as Physical.
Eula Guide: Artifacts
Since Eula gets a bonus to CRIT DMG% as she ascends, you’re looking for ATK% on the Time Piece, Physical DMG on the Goblet, and CRIT Rate on her Circlet. CRIT DMG is good if you have CRIT Rate in the sub-stats.
Speaking of sub-stats, Eula has one key issue holding her back: Energy. You’ll want to grab Energy Recharge in sub-stats if you can and make sure you have some battery characters in your party like Xingqiu (who is also in the banner).
Eula wants to be on the field, and her abilities let her tank through hits by increasing her Resistance to damage and interruption, giving her a form of super armor. Defense in the sub-stats might not be a bad idea, especially if you play co-op often.
2 Bloodstained Chivalry / 2 Pale Flame
Bloodstained gets you a 25% damage bonus for Physical attacks, and the Pale Flame gives another 25%. This combination lets Eula hit as hard as she can.
4 Pale Flame
The four-piece Pale Flame’s effect seems to be made for Eula. When her Skill hits, the Pale Flame gives a 9% ATK boost for 7 seconds. It can stack twice, which is conveniently how many times Eula can use her Skill before it stops buffing. When two of the Pale Flame’s buff is on, the Physical DMG bonus jumps to 100%. This set is harder to get since you’ll be looking for good sub-stats.
Eula Guide: Weapons
If you have five-star Claymores, give them to her. Which one doesn’t really matter, since most players will only have one of these. If you have the Song of Broken Pines from the concurrent weapon banner, give it to her as it was made for Eula.
For free-to-play players or light spenders, there are a few great options for her. On the free-to-play side, Snow-Tombed Starsilver is great on Eula and is one of her best weapons even compared to the five-stars. It can be crafted, and you get the recipe by completing a puzzle in Dragonspine.
It gives a Physical Damage bonus and drops an icicle 60% of the time at first Refinement. The icicle deals 80% of her Attack as an AoE, but will deal 200% against enemies afflicted with Cryo. Since Eula doesn’t work well in a quick-swap team, chances you won’t be reacting with the Cryo that Eula dishes out with her Skill.
The Battle Pass’s Serpent Spine is also great on her, as she’ll be able to take advantage of the scaling damage buff the longer she’s out. Every four seconds, she’ll deal 6% more damage but take 3% more damage in return, up to five stacks. A stack is reduced when she takes damage. The sub-stat on Serpent Spine is CRIT Rate, playing well with her naturally high Crit DMG.