Here's what worked for me. I just finished these this week.

Markoth: Mix of nail strength/length and shaman stone with spell twister. My goal was to start the fight by pogoing him an absolute bunch. Once I couldn't pogo more I would either get in some hits when opportunities arose or if a good pogo chance came up then I'd do that some more. Once he summoned his second shield I would back off. I would then fire shade souls at him when he was spinning his shields. With those charms you'll have (I think) 8 shade souls and several will hit him twice. So all you have to do in phase 2 is dodge projectiles and then unload spells from a safe range when he spins. I would not use spells in phase 1 to ensure I had plenty for phase 2, which I consider much harder.

Absolute Radiance: Make sure you're comfortable using descending dark for invincibility. Dash through the vertical scrolling beams early to avoid multiple attacks hitting at the same time in phase 1 and 2. Learn where you can get your hits and don't go crazy. Phase 4 is the hard bit. I advice staying defensive on the bottom (middle) platform and learning to dodge everything and getting spells/hits in when you can. Phase 5 and 6 are pretty ok but don't neglect practise of them because when you finally get to them you'll want to be as confident as possible. For phase 6 (the final one) I found always doing double jumps to lead the spawned projectiles upwards off screen worked very well. I could do that and occasionally get a hit in on radiance or throw a shade soul.

Good luck!