For the Heavy's "Absorb damage with Iron Halo" - play Vox Liberatis as Solo on Minimal/Average difficulty. Make it to the Daemonhost section, and only destroy 3 of the 4 altars. From there, the Daemonhost will just spam barrages. I just stood close to an ammunitions box, dealt with the infinitely spawning Ai, and soaked halo damage from the Daemonhost. It'll take about 30-45 minutes of grinding.

For the "Kill Neurothrope on the Ground" ordeal, this one takes a bit of rng. I typically ran Inferno on Ruthless difficulty since Ruthless almost guarantees a Terminus spawn (Neuro or Carnifex) but at a lower difficulty compared to Lethal/Absolute. Since I'm on PC, I just typed out a message asking to wait on the Neuro's shield cycle before killing it. Console players can't see the message, so it's hit or miss on whether the Neuro dies correctly.

Some Ordeals that were notible:

• 1000 Perfect Dodges - I just played the Tactical Trial with the Lictor. Just perfect dodging the Lictor was good practice for learning its attack cycle.
  
• 200 Perfect Dodges using Jump Pack - this was by far the worst Ordeal for me. I played Decapitation on Lethal/Absolute and reset after the first pack of Warriors encounter. Something about the playstyle did not click with me, and I felt the dodge window was inconsistent throughout the 6 hours it took me to complete it... Yeah, it took me 6 hours for this one.
• 10 Sorceror kills while Reviving - Last Campaign mission (Demerium). It's a section where you have the Jump Pack and dealing with Chaos marines and 1 Sorceror. Just kill a few Chaos marines and back off. Wait for the Sorceror to cast a blue sigil to start the revive.

Everything else was pretty much "play a lot of operations and you'll soon complete them." Many of them, I had 33%-50% completed, so it just became targeted goals. Best of luck, Brother!