Treat the NPC as you would a Monster
There isn't a direct equivalent for a 25th level cleric correlated to CR, so you need to go through the "how to build a monster" steps from the DMG pages 273 - 283. Likewise, look at "Monsters with Classes" on page 283 of the DMG. It's not an exact science, though the guidance on page 282 says this:
You can create an NPC Stat Block as you would a monster stat block.
That means that you would use the tables on pages 274 and 275, as well as features on pages 280 - 282. It is going to take some fiddling to get the CR "just right" when you combine hit points, offense, defense, spell casting, etc. When you consider that an Empyrean(MM, p. 130) is CR 23, some of its features (like spell resistance) could be earned after level 20 by a PC via Epic Boons (DMG p. 232).
An issue you need to address is the "bag of hit points" feature that increasing CR brings with it. AD&D 2e characters don't typically have 300+ HP. How do you render the character's HP as "effective hit points" - immunities, resistances, and Armor Class are some ways. The AC value in the table tops out at 19, yet many high CR monsters have higher AC than that. These are all ways the NPC avoids getting hit. You can also significantly boost the offensive CR of the NPC. (The Empyrean's spell save DC is 23; see below for more on that).
Alternatively, this is a PC run by the DM; translate based on stats/items
The DMG suggests building an NPC like a PC (DMG p. 282), which points you to the PHB. Start with a level 20 Cleric, and then add magic items, blessings(p. 227-228) and epic boons (DMG p. 230-232) to reflect increased level, capability and challenge. Since the Cleric is a spell caster, boosting the Cleric's wisdom so that save DC's are higher than 20 (table, DMG, p. 274) can render some spells as an "auto success" against players with no proficiency in that save (based on ability scores). That has an impact on the Offensive CR, which you'd average with Defensive CR to get your first estimation of CR. Tinker with this, since Save DC boosts can wander into "save or suck" territory easily.
Example: A proficiency bonus for a 22 CR creatures is +7. With a
Wisdom of 24 (boons/blessings) your cleric would have a save DC base
of 22 (8+ 7 + 7) There is no auto success on a saving throw - any
PC/creature without proficiency in Wisdom saving throws fails the save Hold Person unless their wisdom is at least 14 (+2 bonus) or they have some other magical boost to their save is applied (blessing, ring of protection, etc). Advantage won't help. Save DC of 22 is typically seen in a creature of DC 27, and would contribute to this NPC cleric's offensive challenge rating. (DMG p. 275).
Fiddle with that - the cleric's Save DC, via either stats or magic items - as you combine Offensive and Defensive CR to find your sweet spot.
Consider adding Legendary Actions and / or Lair Actions
Using the Empyrean again as an example, add some legendary actions to this cleric, just as the NPC Warlord (Volo's Guide to Monsters) emulates a high level fighter, but adds legendary actions to make him tougher.
Empyrean can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Empyrean regains spent legendary actions at the start of their turn.
Attack. The empyrean makes one attack.
Bolster. The empyrean bolsters all nonhostile creatures within 120 feet of it until the end of its next turn. Bolstered creatures can’t be charmed or frightened, and they gain advantage on ability checks and saving throws until the end of the empyrean’s next turn.
Trembling Strike (Costs 2 Actions). The empyrean strikes the ground with its maul, triggering an earth tremor. All other creatures on the ground within 60 feet of the empyrean must succeed on a DC 25 Strength saving trhow or be knocked prone.
Consider just the extra attack legendary action for your NPC high CR cleric. The cleric casts a spell, and then casts another spell as a legendary action. That's quite a bit more powerful than the typical action economy's restriction to one spell per turn for spell casters.
A legendary creature can take a certain number of Special actions—called legendary actions—outside its turn. Only one legendary action option can be used at a time and only at the end of another creature’s turn. A creature regains its spent legendary Actions at the start of its turn. It can forgo using them, and it can’t use them while Incapacitated or otherwise unable to take Actions. If surprised, it can’t use them until after its first turn in the Combat. (MM, page 11).
You can also, if you want this cleric to be the BBEG, add Lair Actions/Features to this NPC to ramp up the challenge. A cleric of that high a level is sure to have a base/temple/shrine/chapel/something, so make it a source of power the way a dragon's lair is.
If a legendary creature has lair Actions, it can use them to harness the ambient magic in its lair. On Initiative count 20 (losing all Initiative ties), it can use one of its lair Action Options. It can’t do so while Incapacitated or otherwise unable to take Actions. If surprised, it can’t use one until after its first turn in the Combat (MM, p. 11)
Character details would be helpful here ...
Without posting the details of the character in question, a more detailed answer can't be offered, but the above process ought to serve you in finding a CR that fits a level this 2e level 25 character.
Single monsters or a BBEG with Henchmen?
You may be better off in setting up the challenge such that your 20+ level cleric has some powerful allies (high CR demons or Celestials) such that your party cannot focus fire on a single target.