Boss battle sucks big time. It's hard, but the slightly awkward controls and the fact that shooting upleft does not work makes it even harder.
Great! I can't understand what is wrong with shooting up-left. It seems to work. Please, follow me on Twitter https://twitter.com/sugamenia to talk about it :)
Thanks!
I'm not really on Twitter, sorry ;-)
I tested a bit more for you because I was surprised and both shooting upleft and downleft does not work. Tried it on Chrome and Edge, both did not work. When trying on my laptop (on Chrome) it actually works!!! But, when trying on my laptop with my bluetooth keyboard it does not work. So it is indeed my hardware apparently.
Then, I remembered something about 'rollover' and apparently my keyboard (a relatively expensive Logitech MX Keys, go figure) can't handle specific key combinations when the left arrow is pressed, among which left+up/down and V. So it's not your game, it's my keyboard.
Some info: https://en.wikipedia.org/wiki/Key_rollover
A test tool: https://drakeirving.github.io/MultiKeyDisplay/