Speaking of Oceanid, I actually avoided that boss for the first few months of playing, especially since I focused on leveling up the 4 basic F2P characters (Lumine, Amber, Kaeya, and Lisa). I wasn't using any hydro character, so there was no real reason to fight Oceanid.
Then, out of curiosity, I tried out the Oceanid boss fight. Got swamped by the water illusions so many times and I gave up; I simply could not defeat it without using food items to heal my characters - not so fun to keep wasting food items.
But after I got a little bit bored with the 4 basic characters, I thought about leveling up the other characters I got from wishes. Mona was one of the first 5 star characters I got, so I decided to focus on levelling her up, and thus there's no choice but to keep fighting Oceanid for a while.
And I finally figured out that the boss can be defeated with the 4 basic characters with not too much hassle:
- Distract the water illusions with Amber's Baron Bunny right after they appear. Be careful not to get too close when throwing the doll though, or else they will still focus on you
- When the water illusions are focused on Baron Bunny, keep attacking them with Lisa from a distance
- After Baron Bunny explodes, the water illusions will approach you. Use Kaeya's elemental skill to freeze them
- Finish them off with Kaeya's physical damage
- Sometimes one of the water illusions will become a water time bomb after it gets defeated. You can switch to Aether/Lumine and then run away (quite likely that you'll still be inside the blast radius though, but the point is to make Aether's or Lumine's role as the one who takes damage, as opposed to the one that deals damage)
- Keep repeating the previous steps. Understandably near the end of the fight, two sets of water illusions will start appearing at a go, instead of only one set. Although you may mess up at this phase, where only one set is distracted by Baron Bunny, it's unlikely to the point where your entire party dies
Also, of course, this should be done when your characters are not underleveled (along with their weapons and artifacts), but I don't think there's a need to get the best possible artifacts sets or weapons or whatever out there. In fact, my Amber and Lisa were using 3-star weapons (actually still using the same 3-star weapons now).