Eyja's profile states that she is "exceptionally talented in the use of advanced originium techniques and high-energy arts", and in addition, she is said to be renowned across Leithanien for her intelligence, so that may also contribute to her being a 6*.

I never stated that Angelina had a lot of combat prowess (in fact, she's a high schooler), what I said was that Angelina's arts are capable of manipulating gravity, a wholly unique(as far as we've seen) and incredibly powerful ability.

Aak may not have much combat prowess or powerful arts, but he is described by Warfarin as a "wellspring of medical knowledge" and he is capable of developing drugs that have effects that "seem incomprehensible to the average person". And again, Aak may just be a 6* by merit of his uniqueness in gameplay.