You'll need to start thinking about armored core games differently from souls games they're more like puzzles than skill based. If your mech isnt working out for a boss fight you need to switch up the loadout and if that isn't working out then errr
Gatlings
I had some trouble with the final boss fights but nothing a few stun sticks and gatlings couldn't solve
The build that I used made the game trivially easy. I didn't come up with it and it's probably quite popular. It involves using the heavier double jointed legs, two shotguns and two worm lasers on the shoulders. I wish no build was that strong but it did allow me to get the true ending fast so at least that was welcome.
Ive been using the light reverse joints and basically just upgraded everything else in place. I switch to.double shotguns when i get stuck, and used flamethrowers for exactly 1 mission so far.