Enemies in co-op usually got HP buffs, depends on how many players joined the session. So the more players joined, the more tanky the enemies becomes, even in lower level worlds.
But the question remain: Are you properly level and equipped your character(s)? Because that’s what important in Genshin, where you need to level literally everything, from character level, weapons, talents, and artifacts, which is the most important of them all.
Character level is important, since it can gives you ascension bonus everytime you ascend them. But you don’t need to level all your characters to max, and only your main DPS is enough. Meanwhile, your sub DPS could be left at lv80, while your support at lv70.
Weapons also important, especially when you need to use the ones with the right stats for your character. The most universal stat for weapon is ATK%, but there are people who prefer getting Crit Rate or Crit DMG, since critical can increase your damage really high, depends on your Crit DMG stat.
For talents, you only need to focus at least one of them, or two if you can, depends on how you build your character. For DPS, usually either your normal attacks or skill is the focus, while sub DPS and support is burst or skill.
Now, Artifacts is a whole different matter, since getting the perfect combination (a.k.a best stat and sub stats) is really really hard, since you need to spend lots of Resin to farm them. So unless you’re commit to spend money to farm them, then you can just get at least the right sets, and also the right stats, while the sub stats is only complimentary.
Still about artifacts, you only need to focused leveling 4 of them, which is the Plume, Hourglass, Goblet, and Circlet, while the Flower can just be left at either +12 or +16, unless you want to maximize its sub stats. So get those 4 artifacts to +20 first, since it can greatly affects your character.
Lastly, depend on your character’s elements, you also need to abuse the game’s Elemental Reactions, which can give you huge damage burst. For example, Pyro and Cryo can create Melt, which has 150–200% damage modifier, and it can also proc critical. Same goes to Pyro and Hydro, which will proc Vaporize, with the similar damage calculation like Melt (high damage modifier, and can proc critical).
Another thing that most people often miss is foods and potions, where these things can temporarily increase your stats to a significant degree. So if you feel that your damage is still lacking, you can try create, and use these stuffs to buff up your damage more.