I'll be honest, none of that made sense to me, in terms of how that could be working.
I went and experimented a bit though.
I hadn't spotted it before but when the second phase starts, there's a bizarre delay after the shockwave where the player can fire, but the boss cannot. I'd thought the unique system that gives the boss it's shield might have had a delay set on it for some baffling reason, but there isn't one. Nor on the other systems, but there's still a delay AFTER the shockwave is completely finished.
I tried removing the shockwave to see what happens. Now when the boss changes phases, the shield is *instant*, as is the launching of the spawner shots that summon it's two buddies. So that's one fix... sort of.... it prevents the phase-screen-clear though, which is a problem.
However, defeating the boss without it generating any bullets should STILL be impossible. The thing starts firing the instant the room begins, like any other boss. And I really mean "instant" there. You'd have to be using a weapon with 500+ damage per shot (as the first phase, which summons nothing and uses a gold and blue pattern, has 500 HP as a base, as it's meant to be a short phase) that has zero travel time or simply strikes the entire screen in order to kill it before it has a chance to fire it's first bullets. Actually, it'd have to be more than 500 with the scaling.
Furthermore, Terminus (that's the one you mean, right? Not the Warden, I hope) has 9600 HP as a base when all of it's phases are put together. That's before the game inflates the value for it being in stage 7. Much of that is in it's final two phases, each of which is 3300 HP by itself. Again, that's just the base value. Even if the game didn't inflate the numbers, you'd have to be doing 3300 damage instantly to clear those two phases before the boss can fire. But with the inflation, it'd have to be quite a lot higher, as I understand it. Basically, Terminus's last two phases have enough HP total to equal a normal 2-phase boss.
Either way though, if you're doing damage equal to it's 9600 HP *after* that inflation in order to take it down in under 8 seconds without ANY phase firing (and it has 7 phases), then something is definitely wrong. Nothing should be doing damage that incredibly fast.
I know the thing's HP values aren't perfect, but that's mostly on the enemies it summons (they're a bit too low) and something I intend on changing for the next build. I might up the last two phases a bit as well, but not by THAT much. Like I said, those final two phases are pretty typical for a boss in terms of HP.
If you have that level of damage, no change to the boss itself is REALLY going to fix it. You'd have been tearing up stage 7 without even a threat of taking damage as it is. Even the Flak Launcher, one of the strongest Fearsome types in the game and one that appears in 6/7 only has a max of 1500 HP, if I recall correctly (which is very tanky for a non-boss). So basic enemies practically wouldn't even exist against damage like that.
I cant imagine what must be causing that. One thing's for certain, it's not a thing I personally can fix. As you said, the boss needs to be designed for a fight that doesn't involve the player having ultra high DPS.
Though, here's the question: how many times have you actually fought the thing? I mustn't forget just what genre we're in, after all. It's an Isaac-inspired game, and in this type of game there's always the chance of getting an OP build with a bit of luck and careful choices put together. I don't want to remove the chance of that happening (because it's an occaisional thing that' kinda expected with this type of game, outside of Nuclear Throne). But it should be a RARE thing. And even then, doing what must be 10000+ damage in 5-8 seconds is way too much even for a non-broken OP run.
What mech are you doing this with? What sorts of items are you using, and with what weapons? If you remember that, that'd be a big help to know.
Hoboy. Lots of math here though. Much of which I don't know. I just deal with the enemy side of the game. My job is just to cover the screen in flashy insanity and kill the player a lot, and set enemy stats, and then complain alot. Item/perk/whatever balance is all up to the others.
Ugh, very confusing. All I'm going to do is confuse the situation more if I keep ranting about this. But what I do know is that while Terminus's HP could do with some tweaking... as I had already figured beforehand from my own playthroughs after the patch.... everything ELSE doesn't need changes and tanky things are tanky enough, so I cant do anything there to prevent a player-driven Armageddon in floors 6/7.
Anyone else have any thoughts on this here? Though I'm guessing they'd probably need info about the build you were using to get a handle on what's going on.