If you spam e and left click at about the same time with the shotgun equipped, it will swing the blade and shoot at the same time. This works with all the guns, but the shotgun is the strongest for this strategy. It does the most damage per the interval for interruption. If you run out of amo or charge, 1 defelct of the boss attack and you will refill both. The blood and bright flickering can cause the boss attacks to be difficult to see, and I don't know if there is a setting to change this. Getting hit here isn't too bad as most bosses take 3 or 4 hits to kill shela. You can tank a few of the bosses attacks before needing to refill health. The secondary AR amo can be equipped to do the most damage while out of range for the bosses (most useful on newgame+). I was able to kill some of the bosses so fast that the game didn't load the next area for some time. This works best on new game plus with a maxed out blade. The two dog statues can be killed with 1 clip of the shotgun when they spawn and the bosses are really quick too. Like I said the game didn't know what to do and I was standing around waiting for enemies to load after killing the giant. This is a good time to reload. This might be patched soon given the daily patches the developer is pushing now.

This is the basic boss fight strategy used throughout the current runs with some slight adjustments for Best patch with the 20 round clips. When the devs first added 20 round clips it removed the 1 shot added when you block or press e with an empty mag which was a strat that was used to save ammo. The perfect parry is known to refill ammo and charge already and looks to be intended by devs as it's pointed out in a hint. I see you said this 4 days ago which is just after the current WR the 23m 27s run used strats like this, it's good that you noticed it too without anyone telling you so maybe you'll bring us some amazing runs too!