Power is always good against bosses, that's true, but dude, Wang Yi has all these crazy cancels + speed. She just can't be touched, ESPECIALLY if you're spamming her EX attack. That literally does all the work for you.
Square > Triangle > Triangle (EX Attack, it's the one where she runs forward slicing the fuck out of everyone on her path) > Double air jump cancel (gives you invincibility frames) or you can R1 that shit (the special attack that takes up musou) and cancel into char switch, back to Wang Yi, and then start over with the Square > Triangle > Triangle string. For big bosses I tend to to max out her musou attack (the one where she grabs you, takes you up to the skies and fucks you up) cuz it's pretty much an unblockable and it does about 70% damage :p
And this is pretty much with nothing equipped, if you give her the stun lightning or the unblockable specials... yeah man, I think she's OP lol but yeah you're right, IF she gets hit she flinches, and her strong attacks dont hit as hard as other characters, but one thing I noticed is that EVERYONE goes for Lu Bu