Actually, Belgriff is probably A rank to low AA rank at best. I read the novel raws upto the part where he meets the elf princess.
Sasha is actually better than him, he only beats her due to her inexperience and unfamiliarity to his weird movements caused by his peg leg. Angie sympathizes with Sasha over this. During their 4th(?) spar, Sasha is actually dominating and was close to beating him but got too eager to go for the kill that Belgriff managed to take advantage of the opening. Belgriff knows the next time they spar, Sasha will win and the illusion of his invincibility will thankfully be over.
As for Angie, he actually beats Angie easily because he knows all of her bad habits. Angie was also complacent, thinking she didn't need to get rid of those bad habits as long as only Belgriff knew them even after Belgriff insisted she try to get rid of them. Belgriff was so worried about her not caring about it even after he kept telling her not to be complacent that he actually forced Angie to improve by challenging her to a fight and saying he would disown her if she lost. Angie goes into angst mode and can't fight properly because she can't believe Belgriff would say such a thing. After she gets over it, she beats Belgriff easily then demanded a hug.