I think it also really depends on the player.
I'm used to really, really difficult games (ones that are WAY harder than this one), and used to dodging 8 squillion bullets at once (danmakus). As such, this game doesnt give me all that much trouble, even on the highest difficulty.
Playing on either Master Hero or Chosen One (currently Chosen One), I still very rarely "snipe" at any boss. If it's not on the screen, I'm likely not firing at it.
And beyond even that, I tend to make heavy use of some of the low-range spells. The Brimstone Cyclone, for instance. Even on this high difficulty, my main attack against it is actually Insect Orb, or Ice Cross if it has Earth resistance. I dont bother engaging this boss from a distance.
That's not to say I dont use the straight-shot spells like Ball Lightning and whatever, obviously I'm gonna use those.... but I tend to also use alot of the funkier spells, and do just alot of melee in general. In all honesty, I cant think of any bosses that I'm not at least somewhat close to while fighting. The stupid Elder T-Rex, for instance. I'm usually *really* close to him during the entire fight. Two Rexes worth of distance.... no further than that, often closer to him. Even his "holy crap lotsa dinos" attack isnt THAT bad, if you know what to do. A quick shield and a burst of Insect Orb generally clears most of those dinos in one blast; if there's some leftover ones I'll run around and shoot them, before resuming my position in front of the boss.
Also, I should note..... I dont abuse the shields. This is all done by dodging. Even at a close distance, even on Chosen one, most attacks CAN be dodged. If I shield, it's a quick one-second burst or so, which is how I've always used them.
These tactics do not change if there are multiple bosses, or perhaps boss plus slimes, or in some rare cases, multiple bosses AND slimes. Or blades or whatever. I'm impatient and aggressive, so it wont occur to me to NOT charge at the boss. Even in danmaku shmups, I tend to try and point-blank everything.
Buuuuuut..... the reason why I can do this at all, is because I actually try it. It's not true that there is no reward for attempting such a strategy. The reward is that your own personal skill increases. If you just sit back and snipe.... yeah, you're not gonna get much better at it, and you'll be less likely to survive one of the "meaner" encounters, when the game sticks you in a tiny room with something horrible, as it likes to do every now and then. Dont get me wrong..... this isnt easy to learn to do. It's the sorta thing that takes real practice, and you're GOING to die about a billionty times in the process, same as with learning a shmup. But it can be done.
As it is, I think the boss fights are some of the best parts of the game, particularly considering how the difficulty of a given fight can change based on what the RNG does. The room shape, the enemies that are spawning in the room, stuff like that, can randomly make an easy boss into a really, really hard one, or a nasty boss into something that you just wreck immediately.
Now, that being said, I agree on some points. Some boss rooms are too big. Some of them really seem like they're designed for MULTIPLE bosses, but the game will still select them for single bosses; this leads to the type of boss room where, upon first entering, you have to actually seek the thing out, rather than just start fighting right away, which is kinda annoying. Sniping might not be as much of an option.... and therefore, players might not rely on it so much..... if the rooms were properly selected for what they will contain, one boss, or many.
A couple more points:
I dont think AVWW needs on-hit invulnerability. In all honesty, I'm glad it doesnt have that. It'd DRAMATICALLY reduce the challenge, and it'd screw with alot of numbers. If I get hit by like 30 fireballs at once..... yeah, I should take ALOT of damage. It's 30 freaking fireballs. It shouldnt just damage me for ONE of them, and ignore the others because of magical blinking. If I dont want to take that kind of monstrous damage, I should, perhaps, not allow myself to get hit by 30 fireballs in the first place. And heck, even then, the game is being generous. This game takes alot of inspiration from the shmup genre, and the thing about the shmup genre is..... one hit, you're dead. Just one. Even in a danmaku game, with 90 squillion bullets coming at you at once..... just one kills you.
I also agree that there's too many homing attacks. It's not that they're even difficult to deal with.... frankly, I think most of them are EASY to deal with; bosses that use this attack type too often, are usually bosses that I can wreck really fast (and not from a long distance). I would indeed like to see some more variety here. Now, there IS variety.... you've got things like the Elder Rex and his direct fire spam, the Amoebas and their various spreadshots, the big green frog dude and his horrible exploding green things that fill the area with a million shots, or stuff like the Brimstone Cyclone and it's wacky curved shots and repulsor field. But yeah, too much of the homing stuff.