I love it, one of my favorite submissions so far! Played it a few times until I got S rank.
It's simple, it looks good, it plays good. Great use of the theme too! Enemy bullets felt a little too big but it's probably just me, and while the looping audio is the only glaring weak point in my opinion, it didn't stop me from playing. I noticed some people saying that the last attack is too difficult, and while I noticed how much harder it is than the rest of the patterns, it felt just right to me (though I'm used to playing very difficult bullet hells, so I understand why a less experienced player would think so). Well, maybe the frequency at which the boss spams blocks could be tweaked a little, but it was an exhilarating phase for me regardless.
It's funny how the game prevents cheese and using the blocks defensively by making the boss shoot through them if you let him fill the arena too much. I also really like the choices you get with your upgraded weapon - you can take it slow and break the blocks while keeping your distance from the boss, or you can risk it and pointblank the boss for a faster kill. A feature I personally could use would be a "speedrun timer" to let you see how well you do in the speed regard.
It's really fun as it is, and with some extra work put into it, War in Heaven could definitely be shaped up into something incredible!