Before lvl54 you can use Barrage on Straighter shot as the 3 hits will be guaranteed crits, unless you're very lucking getting 3 crits on a barrage-heavy shot it will net you more damage, pretty useful when doing rouletes and other lvl 50 duties, but it's proc dependant so the rule Hyomin stated is true for this aswell !

Concerning the opener you can found the video here, it's detailled in description and Miyuri answers a lot of questions in comments, a very good gal !