How can a BBEG deal with a Wall of Force and Sickening Radiance spell combo without trivializing it?

This question has already been asked in general form here, but was closed as too broad: "How to deal with broken combos?". I’m going to make an effort to make the scenario I’m asking about more specific to keep my question open.

The spell sickening radiance (XGtE, p. 164), cast just before a wall of force (through a readied action or something similar) can trap and kill many solo Big Bad Evil Guys (BBEGs), with just two casters. This combo is particularly lethal because of the levels of exhaustion the spell gives, but could work even with other persistent damage spells. Sickening radiance is a spell that creates a persistent area of effect for 10 minutes that deals radiant damage and gives exhaustion on failed saves.

The combo works like this: wall of force (WoF) caster readies their action, with the trigger "cast WoF on the BBEG when my friend casts sickening radiance (SR)". SR caster’s turn comes, and they cast SR, centered on the BBEG. WoF gets cast on the BBEG as a sphere, trapping the BBEG within the wall of force for 10 minutes while they get exhausted to death.

What actions can I, as a DM, take to make a fight with a BBEG not as trivial, while still rewarding the two spellcasters’ ingenuity?

I classify "rewarding the two spellcasters’ ingenuity" as at least forcing the BBEG to spend a limited resource to deal with the problem. So a BBEG having disintegrate is one option for what I can do, but giving the BBEG a teleport as an action (or legendary action) is not.

The BBEG wants to delay the party in the final battle… (narrative delay?)

I have thought of a few methods they might accomplish this. In this question, I am wondering about a potentially more controversial option. I am considering having them attempt to monologue or engage the PC’s in dialogue (e.g. offer for them to join his side, explain his plan, answer questions, etc) with a timer running, then increase the battle rounds based on the amount of time they were able to enthrall the PC’s with their speech.

I can see some problems with this:

  • I haven’t used this "on the clock" method before and the players may not recognize what’s happening (kind of the point?); I’ve loosely enforced limiting speech to your turn to maintain a suspension of disbelief
  • This could cause balance issues with planning the time depending on if they catch on quickly or not as all (see below)
  • It would require significant setup, coordinating clues that may let them read into what is happening while also maintaining a monologue
  • Our game is online, so it may be more difficult to implement this strategy

Some details about the campaign/fight (major spoilers for Paizo’s Ruins of Azlant AP)

The party will already be in initiative when they arrive in his area (they have to fight or bypass a couple ‘mooks’ before reaching him) but in our circle of gamers there is a precedent for NPC’s to interrupt combat rounds with speech. In this campaign, that even includes the players taking up another potential enemy’s offer of working together instead of fighting. In the past, however, it hasn’t been relevant that the clock continues moving.

My goal is to cost them a handful of rounds if they are willing to listen to the creature, who is manipulative by nature. I feel this is thematic but unlikely to make the difference in their success or failure in regards to the creature’s plan, but I think it would be an epic moment if it does (or comes down to the line).

Is this a bad idea? Has anyone had success doing something like this, and what was needed to pull it off?

Directly related to my question about doing it with spells.

Reminder: per "Good Subjective, Bad Subjective", to answer this question, one would either need direct experience using such a delay tactic or have experienced a GM doing so with them as a player.

The BBEG wants to delay the party in the final battle… (spells)

I have thought of a few methods they might accomplish this. The easiest, mechanically, is to give him spells to accomplish this.

Some details about the campaign/fight (major spoilers for Paizo’s Ruins of Azlant AP)

What are the best 5 arcane spells for area denial? The area to be protected is a control panel that can be reached from two adjacent squares; there is also a hallway leading into the room that they will need to travel through that is 10×10 and 40ft long.

Some members of the party have inanely high Saves, especially against spells, so spells that do not require Saves or still have some effect with a Save should be considered stronger than similar spells that do. The party, as most high level organizations do, have easy access to Freedom of Movement.

Directly related to my question about doing it with narrative.

How Can A BBEG Mainpulate Tiles With Anti-Magic On It’s Turn?

My players will be fighting a BBEG tonight that they will have no choice but to face in its lair. Part of the lair design is that each tile in the room is anti-magic and I wanted the BBEG to be able to manipulate which tiles are or are not activated during its turn.

Could the BBEG use item interaction to telepathically control the anti-magic tiles?

Is it okay to have a DMPC who is also the BBEG?

I don’t want to make my DMPC star of the show by any means, I realize how important it is to let the characters do their own thing. I want this character to act in the background and support the other characters from the side, not for party benefit, but because it benefits the DMPC character. The idea of having this subtly manipulative character influence the party for their own means and to quietly gain power from it intrigues me.

I want to have a grand twist where this character I’ve made betrays the party, and they realize that they’ve let themselves be led astray from their original task just a little bit too late for them to do anything about it. But by defeating the DMPC, they will also and up accomplishing their original goal.

For context, this campaign is set to take place in an off-shoot version of hell where many people are given free roam. So it would be kind of normal to have betrayal there.

I just want to know if it would be possible to pull this off.

What is the CR of my BBEG? [on hold]

I am creating the Big Bad Evil Guy for my campaign and I don’t know what the CR would be.

BBEG

Medium humanoid , lawful evil

Armor Class 22 (Natural Armor)
Hit Points 285 (2d8/18d6+200)
Speed 30 ft.

\begin{array}{cccccc} \text{STR} & \text{DEX} & \text{CON} & \text{INT} & \text{WIS} & \text{CHA} \ 10\;(+0) & 13\;(+1) & 30\;(+10) & 30\;(+10) & 14\;(+2) & 16\;(+3) \end{array}

Saving Throws Dex +7, Con +16, Int +16
Skills Athletics +6, Deception +15, Insight +8, Intimidation +9, Persuasion +15, Religion +16
Damage Resistance magical damage Senses passive Perception 12

This is a Rogue 2/ Wizard 18 PC turned NPC bad guy. He has a spell attack bonus of +16, and a spell save DC of 24. His stats are granted by magic items. He knows all magic spells available in D&D 5e and has most of the known magic items in the game. I can’t figure out what the CR would be. Can anyone help? What is his CR?

How can I make my BBEG immortal short of making them a Lich or Vampire?

I’m trying to create a BBEG that can’t die naturally, or at least lives for thousands of years. The PCs need to be able to kill them in the end though. I know I can use a Lich or Vampire but is there any other way to accomplish this?

I’m not interested in monsters who are already immortal, I am interested in finding a mechanic that will allow my BBEG to be immortal regardless of their race. I am considering making BBEG a Loxodon, but have not yet settled on anything. You can assume that BBEG remains on the material plane for the duration.

The final battle will be at about Level 15 for the PCs.