Here's my current build...last night's was a bit different. I forget what I changed.
https://imgur.com/a/blzyVxf

There's still stuff I need to do to further optimize it, but it's doing pretty well so far. Hopefully, they don't go crazy and nerf DOTs then next patch...

Yes, you are right. The first thing that I thought of was: they are gonna nerf DOTs (now that they seem to be good).

Here is what I did (this part is pretty standard):

2) destroy the shield generator (I don't think it matters when you do this, as long as you do it before you take on the Knight)

3) go around the map and kill off the human enemies/turrets/guns/emplacements
4) grind away at the Knight

Here were my initial problems (before making numerous changes to my build) before finally defeating this POS:
1) Neocore totally neutered my old build, which was capable of doing +14s EASILY. They nerfed 3-shot, power armor DR, +2 sec invuln. on kill power armor enchant, diversive tactics, % dam/debuff, AB2, and Stride of the Blessed perk (-10% DR instead of -5%...as if adding +5% MS made up for that...lol)
2) I wasn't doing enough damage with my old build (read point #1)
3) I was taking WAY too much damage (read point #1). This included losing almost all of my health from random Chaos Marine bolter shots; getting 1-shot by random attacks from the Knight; getting molested by the rocket turrets; getting molested by the flamer turrets; running for my life when the Knight decided to start using the autocannon; etc.
4) my sustain wasn't good enough

Here was the problem later on (after making changes to my build):
1) I would take out everything except the Knight, but I couldn't kill the Knight. I mean I literally could NOT kill it. The blue shield never went down, no matter how much I attacked.

Solution to the last problem:
1) I made more changes to my flamer build...I added more and more damage to it (through re-rolling).
2) I started using left click when it was just me vs the Knight. The flamer's LC skill seems to reduce the Knight's suppression. I "stun" locked the piece of shit until he finally died. So, what I would basically do was hold LC, 1+2. This made me overheat, but I didn't care, as long as he remained useless. Later on, I mixed it up with 1/2 rotations, and some LC mixed in. Then I went back to LC+1+2. Basically, I had my way with him...but the grind was soooooooooooooooooo tedious. I had to attack it for what felt like 20 min+. ALSO, twice, some Chaos Marines and demons joined the fight during this 1v1 battle. I either forgot about this, or it's a new mechanic that Neocore added. Before the patch, this never happened to me because I would take out the Chaos Knight so quickly (a few seconds at most).

I will post my build in a bit.

Hey man, I'm not singing a different tune at all. I just felt bad the way I insulted them in that one post. The changes they made are moronic and lazy, and nothing will change that...but...I was happy that I beat that scumbag knight.
Also, have you done these missions after the seasons patch? Before, these missions were easy.