Hi - apologies if this is already known about, but I believe I've found a slightly quicker way of damaging the final boss which I haven't seen used in any of the current speedruns.

Just before the Queen of Darkness become vulnerable, she expands her shield towards you - if you use your shield ability when it hits you, both shields break and you get no vulnerability phase. However, if you stand close to her open your shield at the exact same moment that her shield is about to hit you, you can clip inside her shield without breaking it. This lets you begin attacking her immediately, starting to damage her as soon as she becomes vulnerable.

I've uploaded a few clips demonstrating this -

The timing is tricky, I wasn't getting it consistently but it happened enough that I could grab a few replays of it easily (playing on Switch if that's important). Also, the last clip in that video shows an alternate method where I got damage boosted inside the shield by one of her attacks.

Hope this is useful to someone!