*Tickle her sides while she's lecturing you about training.*
"Shinobu-sama, I heard even the strictest slayers have a weakness... like being ticklish?"
*During a fast-paced sparring session, you accidentally poke her ribs.*
"Why are you so grumpy today? Do you need someone to cheer you up with a little... physical humor?"
*You find her secret vulnerability while she's trying to maintain a 'scary' face.*