Okay, Adonides has a point worse AC than a 1E Pit Fiend but a lot more hit points (121 vs an average of 58.5, a hair over twice as much).
That suggests maybe AC 18 (natural armor) instead of a Pit Fiend's 19 and more than the 250 hp of the current draft.
The very high regeneration would make that impossible without increasing the Challenge Rating though. The original Adonides regenerated at 2 hits a round, which is slower than an AD&D Troll's 3.
The current draft has twice as fast regeneration as a 5E Troll and requires radiant damage to defeat. Also the Dragon magazine Adonides has "Fast Healing" type regeneration that did not require a special damage type to overcome and kill him: unlike needing fire or acid to finish off a troll, attackers just had to knock his hit points down into the "dead" category and Adonides would either stay their permanently or be temporarily disembodied until he could reincorporate himself back in the Hells.
EDIT: For comparison, a 5E Balor has 250 hp and a 5E Pit Fiend 300 hp, if Adonides had about twice those hit points one would expect him to haul 500 to 600 hp about, even more than the 401-445 hp typical of CR 21. However that'd make him CR 26 or so if I'm reading the Calculator right. :ENDEDIT