Invisibility guide?
Toggle them on or off? If they’re on that’s bad. If you turn your camera during the heartseeker animation, as soon as it ends your character will instantly turn to face the direction the camera is facing (as long as right click is held down). It’s important to wait for this or you’ll go flying in the wrong direction. On your 4th leap, don’t bother waiting, as it doesn’t matter if you don’t end up in the circle. Try to do your leaps through the outer part of the circle and go around the circle. With daredevil it’s also possible to do a 5 stack instead of a 4. Dodge back, forward, back, short heartseeker, heartseeker. You can also try holding back instead of strafe. It works as well, and may be more comfortable.
Yes, turning the action camera off is important. From my last experience with it, it was programmed to not allow you to shorten your Heartseeker distance. So make sure it’s off.
And also, just in case you didn’t, you have to deselect any target before you Heartseeker. If you have any target at all, you will leap toward it.
I would suggest practicing your leap combos without using Black Powder first. Start out slow. Practice leaping once, then pause as you turn the camera, then leap again. If you do successfully leap in the correct directions, then shorten the pause between the two. Do that until you find the minimum pause needed between leaps. Then add a third leap, then a fourth. After that, add the Black Powder at the beginning so you will actually enter Stealth. It definitely takes practice.
As for what that one Thief did, it could have been a couple of things. He could have used Shadow Refuge which is a longer-lasting Stealth utility. Or he could have used Black Powder from his off-hand Pistol, then switched to Shortbow and used Cluster Bomb inside the circle several times.
Black Powder + Heartseeker = Stealth
Black Powder + Cluster Bomb = Area Stealth