Should I level my Bryhnild's second skill?
She's currently 10/6/10. This event is the first time the skill has been kinda useful, is it worth it at lvl 10?
For general purpose, no. It's not worth it. As the other guy said, the skill is very situational. Nice to have when its needed, but you're not gonna need it all too often.
That said, if you like her and just want to see her at 10/10/10, don't let us stop you. I 10/10/10'd my Tamamo without any regrets.