Is anyone beyond Teshuva? Surely Hitler could not have been forgiven if he did teshuva?

Only God is the real Forgiver, and He is the only One Who knows and decides. But regarding your extreme question, the Talmud recounts severely evil gentiles (Gittin 56b) who are not enabled to repent, and accordingly surely Hitler ym"sh definitely could not repent. On the other hand, most rabbinic sources, especially those relating to Teshuva, deal with Jews, and the sources go out of their way to stress that even when our sages teach that a certain sin cannot be forgiven, that is just part of the challenge. When Elisha ben Avuya is told, "all can return except him" (Chagiga 15a), R. Tzaddok & Rav Kook explain that if nevertheless he returns, it is totally sincere and without ulterior motives, and accordingly is definitely accepted.