The way I understand enemy targeting is that they have an AI, so they try to do smart things like attacking weakened or surrounded characters. And then there is a layer of RNG on top of that. So something like Phoenix Dance moves the RNG slightly in that character's favor but it doesn't completely override the AI. That's why I don't really like Phoenix Dance. Using cover skills is a much better way to protect your fragile characters IMO.
The "weapon skills cost less" formations are really good in my opinion if you have one character that is your main damage dealer. Like axe for example has a lot of powerful high BP attacks so the axe formation is pretty good.
Overall I would say formations aren't *too* important though. I used Royal Hearts exclusively for like 70% of the game before I finally started experimenting with them.
Well Royal Hearts is still my default too, so that's good to hear, haha. I should start messing around with these weapons ones more soon; I'm just still not always great at picking someone who will actually take down the enemy group well, even with the loading screen tips...
Also, I liked some of the "kill/unison = more BP" ones early on but they feel like such a big risk too? Even in fights where there's a decent amount of little enemies you want to kill before the big one, it's a big ask to bet on getting them ASAP, cause if you can't you have too low BP to do much. Especially when you're stuck killing without a unison because keeping them around would be too dangerous...