Which items work well / don't work for Boss battle's?

[WhYdOyOuThInKtHaThApPeNeD]

Seriously crazy, please check this out, I really want to know what's the deal!

So I did an attack run on the Driller with 1 Troll stack. Everything exactly the same each time except 1 time with the Cloak of Shadows, the next time without the Cloak of Shadows.

Here is the damage done by the Driller on first run with cloak:

347
878

355
412

912

368
929

384

379

364
941

346
942

Here is the damage done by the driller after reloading and without the cloak

347
878

355
412

912

368
929

384

379

364
941

346
942

So apparently the cloak is useless for the Driller and probably other bosses.

But this really proves something else I've been noticing. Certain things don't seem so random. It's impossible that the damage done by the Driller on both runs would coincidentally be the exact same for each hit for every single hit.

I've seen this in many other cases. I often play the same battle over again several times. Sometimes to get a no loss or in case something really bad happens. But usually I do it just to try different things to learn how things work and because some battles are fun. I've got a whole collection of save points just before good battles.

Sometimes I'll do the exact same first move in a battle. For a Red Dragon once I remember it did the exact same damage every single time no matter how many times I did it. Got the same critical hit every time.

So what is up with that?