lvl up euryale for Gawain fight in camelot? and which others?

Euryale is more-or-less the perfect boss killer for Camelot given that just about everyone is a male Saber.

If you don't have a Lancer with more stars already, it's technically possible for Leonidas to solo Tristan. Otherwise supports that aren't lancers for your Orion/Euryale.

Robin Hood is also a good choice, if you have other more archer, go use them~

Artemis destroys Gaiwan