Judgement Day was a pain really for a couple reasons. First, you can't cast more than 2 guardians at first, which means the second you stop summuning to stack fortresses, the king will jump on you, ruining your strategy for a couple seconds while you try to summun back into control. Edyrems on medallion might help for this.
Second, the damage output. Tu Labrys STRONGLY recommended here. Deathgaze does very little damage compared to Iceburst. ( Started with Deathgaze because I thought AR would be an issue, but my 24k AR cut through avatars like butter)
Lastly, once an avatar spawns, kill it asap. But prepare to be swarmed the second you stop casting because Guardians/Edyrems die in 1 hit. I did manage to get upgrade first try, dying on the first avatar almost killing him. I told myself: Meh, I'll finish him off and just grab charm. While finishing him off, a second obe popped, which I managed to kill while being Tremor'd to no end. My hp was at 93 when I got back town. Should have got that on video, I barely believed it myself.
Belial next, I never liked this one very much. No cheating on this either, only thing I did was twinking Emp Bands and Teganze. Overall easier than I thought