Shovel Knight: Specter of Torment

I found the easiest way to do this trophy/feat is with the Barrier Lantern. Which boss you go for is really up to you but I recommend either Plague Knight or King Knight. At the start of the fight, activate your Barrier Lantern. The Lantern costs 1 bar of Darkness to use. The flames will stay active as long as you don't take any damage, without costing any additional Darkness. If you press your normal attack button you will send your barrier flames forward to perform an attack. Although you can use this for additional damage, it will cost more Darkness to re-summon them. I prefer to use the flames to do damage while they are still acting as a barrier. While the flames are acting as a barrier around you, they can actually block some projectiles. As for a general rule of thumb, if Specter's normal scythe attack can destroy a projectile, the Barrier Lantern's flames probably can as well. There are also things that the Lantern can block that your scythe can't.

If you are having a bit of trouble with having enough Darkness, I can recommend 2 things:

1: Try again after you have acquired more Darkness Wisps. You can collect an additional 3 Darkness Wisps, increasing your maximum Darkness to 5 bars.

2: If that is still not enough, you can always try using the Raiment of Risk (see Spec'd Out), as your Darkness will slowly regenerate over time during the fight. However, just be aware that your max Will will be reduced to 4. If you are taking a bunch of hits, don't be afraid to switch to and subsequently use, the Will Skull to get some health back.

I found this to work on any of the main stage bosses, including Black Knight on the Plains. I didn't bother trying the final 2 stages as I don't think they fit the criteria in the trophy/Feat description, as well as how difficult it would be to succeed.

Here is a video with examples for King Knight and Plague Knight: