Destroyer and it's instant kill attack...

It starts to vibrate before it fires. You have two or three seconda window before it charges.

If its dual cannons are still active and you don’t hear them firing, assume that it’s targeting someone with its super cannon. I always try to account for a Destroyer(s) location at all times when I am fighting Remnant.

Some characters fully shielded can survive the initial blast but not the second. If you get hit, quickly find cover before the second blast. If not, hope someone is nearby to rez you.

The chest opens up in a very clear and obvious way maybe ~4 or so seconds before the first shot, and then the 2nd always follows exactly 2-3 seconds after.  There's usually time to react and boost out of the way or take cover in between the 2 shots.

There's also an audio queue that I suppose you might be missing if you listen to music or mute the game sound.

Not sure how you are missing the giant glowing ball of light in it's chest in the seconds before it fires, or the sudden stopping of the machine guns (which is another tell it's going to shoot). I'm not trying to be snarky at all but maybe try looking at it?

Yeah, they take a long time. Wait for the mouth to open before unloading on it. Or, like with Destroyers, take out the turrets first before tackling the body.

During Boss objectives, I do tend to shoot 2-3 Cobras to make it more manageable. If time is running out, I'll use all my Cobras.

Or the fire barrels!

Before they patched that I used 90-95% of my rockets on Destroyers.

After the patch I pretty much just stopped using them.

Yeah, same here.

GitGudSkrub Xexexe

Remember the good ol' times when you could spawn kill everything with the Cobra on wave 11? oh, wait!

_{Use a Blackwidow or a shorty}

Since when am I 'some ppl'?