@Eppie
To easily beat Oceanid you really need to use support build
Fischl (Thundering Fury 4 set). Favoniuos Warbow is good enough for me due to Energy recharge. Her raven OZ is absolutely killer for water birds. With the support of cryo and fire characters. I used Noelle (max defense) to provide shield/healing. And if it still too hard you could use 25% hydro defense potions - built at the alchemy station.
I learned one trick to make the leveling grind a bit easier - use Condensed resin instead of the original one.
That gives you a 2x reward (typically with a better chance too). So 2x (better) reward for 2x resin while only need 1x effort = (a small) win