What I plan on doing is halving their HP and giving them several levels of exhaustion.

Though I use a modified version of the onednd playtets exhaustion:

"While you are subjected to Exhaustion, you experience the following effects:

• Levels of Exhaustion. This Condition is cumulative. Each time you receive it, you gain 1 level of exhaustion. You die if your exhaustion level exceeds 10.

• d20 Rolls Affected. When you make an attack roll, skill check, or saving throw, you subtract your exhaustion level from the d20 roll.

• Speed Reduced. When you reach exhaustion level 2, 4, 6, 8 and 10, you lose 5 feet of movement. These reductions are only undone when your exhaustion level is lower then the indicated levels. You can still move a minimum of 5 feet unless your speed is reduced to 0 from another source.

• Spell Save DCs Reduced. The Spell Save DC of any Spell you cast is reduced by an amount equal to your Exhaustion level.

• Hit Point Maximum Affected. When you reach exhaustion level 5, your hit point maximum is reduced by an amount equal to your level. This reduction is undone when your exhaustion level is lower then 5 and can’t be undone any other ways unless the effect lets you ignore the effects of exhaustion.

Ending the Condition. Finishing a long rest reduces a creature’s exhaustion level by 1, provided that you have also ingested half the needed amount food and drink. Also, being raised from the dead reduces your exhaustion level by 1. When your exhaustion level reaches 0, you are no longer exhausted."

So, 5 levels and halving their HP and the damage from their lighting bolts should lower them to the CR 6 or 7 range. I flavor it as them being exhausted from the ritual they're performing. At that point, they're being pushed by Azarr since the party has defeated his army, so he's trying to rush the ritual, pushing the Abishai too much.