I had a very similar experience with one of my early instructors when I was a red belt. He's a very big guy and very imposing. Not necessarily tall, but big... lot's of presence. He used to be the only person I never seemed to be able to score any kind of hits on. What made it even harder to hit this guy was the fact that he would just take a stance and not move. Then, when you started trying to advance, he'd just put on this huge grin and say something like, "sure you want to be there? You're in my range." ( drove me insane.)
Well after sparring one night he told me something. "You're a red belt now. You should throw everybody that stands accross from you on every sparring night to help them learn.. everybody. He wouldn't even allow me the slightest whine about not being as good as the bb's.
Next sparing class comes and I'm determined to try. I actually do throw everyone in the class,,, until I come to HIM. There he stands grinning. No attacks to counter; he's making me attack. So, finally I just stand off with him. I watch until he makes the slightest flinch and I just blast in for a throw. Head tucked into my shoulder, left hand high cover, right hand low, and eyes completely closed expecting to get knockedTFO. To my surprise, he doesn't give me a kick to throw him with but that lead left hand connects right on the chin... perfect. He takes two steps back, feels his jaw and regains his composure. AND, his grin turns into a full ear to ear smile. "GOOOD ONE!, LET'S GO!"
I bow and say so sorry sir. He says, "Why? For What? It's all good" So I get back in stance and so does he. Now, out of nowhere this really large man moves instead of being that mountain. He jumps in the air and does a scissor takedown on me. Not the nice one leg behind and hold on and drag the opponent to the ground. He slammed his right leg into my gut and swept my feet with his left. We hit the groud with a thud so loud the entire class stopped sparring and looked our way. I barely avoided having the wind knocked out of me but man it hurt.... and I survived. He helps me up and whispers quietly, "that's for the jaw." I say "Yes sir." After that, it was fun. It was ALL GOOD. Definately no better way to bond with an instructor than a good kick in the face.
And I've never been afraid to spar anyone since. I don't say I am better than everyone, but I've just never been afraid since. I didn't always win an encounter or sparring match,,, but I always left my opponent with something to remember me by.