From what I know, Beato didn't want to fight Battler because she liked murdering; she fought him to find the truth behind it all. Battler still doesn't notice that, though, so in his eyes he sees Beato as Beatrice, his enemy. I doubt Beato sees him the same way, so I guess I'm up for Bern.

Shannon can't be her main enemy either *SPOILER* since they're the same person after all. */SPOILER*

Battler and Frederica

^^^ Exactly!

In my honest opinion , I believe that Bernkastel is easily Beatrice's greatest enemy. Some of the reasons being :
1) In the Meta-game world scene after EP1 , The time when Bernkastel is introduced ,she announces that she is the cruelest witch in the world and was only helping Battler to 'stave off her boredom'.
2)In the extra TIPS titled,"The Witches' Tanabata ,it is shown that Bern was responsible for fueling Ange's hatred towards Eva and Beatrice.{Sorry for the spoiler.}
3)Starting from EP5, she begins to use methods to destroy Beato's existence and the chances of her ever geting a happy ending altogether.etcetra.
I guess that's why Bern is my absolute favourite character from Umineko.She is sooo delightfully cruel,cunning and twisted.Kihihihi {I'm copying Maria, ~ uu ~ LOL}