Harm the casters, break their concentration

The BBEG might have some way to harm the characters without attacking them directly. BBEGs regularly have minions (and if your BBEG doesn't have minions, why not?). The BBEG would order their minions to focus fire on the two casters, attempting to break their concentration.

Until such time that concentration is broken, though, the BBEG is trapped, unable to fight directly, and making saves against sickening radiance. Depending on circumstances, it might be many rounds before the BBEG can escape. This rewards the party's ingenuity.

Fight magic with magic

There are many spells available to spellcasters to either stop or suppress the spells in this combo. These options require the BBEG to use up spell slots (potentially high level spell slots), and many of them will require the BBEG to be exposed to the combo for at least one round.

If the BBEG is not a spellcaster and does not have magic items which can cast some of these spells on their behalf, their options are sorely limited.

Antimagic Field

The nuclear option for high-level spellcasters fighting against other spellcasters. This will allow the BBEG to completely ignore both sickening radiance and wall of force. However, it also suppresses any buff spells the BBEG may have had active and prevents them from casting other spells, although they can always drop concentration on antimagic field once they are clear of the combo. This costs an eight level spell slot, though, so is a very costly counter to the combo.

Globe of Invulnerability

This spell has a similar effect to antimagic field, in that it completely negates wall of force and sickening radiance, but it uses a lower level spell slot (although at 6th level its still significant) and doesn't stop the BBEG from casting spells. But the casters can also keep casting spells at the BBEG by moving to within 10 feet of the BBEG.

Etherealness

Wall of force prevents escape via the Ethereal Plane, but side-stepping to the Ethereal Plane allows the BBEG to ignore sickening radiance. Etherealness's 8 hour duration allows the BBEG to wait out the spells of the combo and then move into a more advantageous position (or possibly flee). This requires the BBEG to spend a 7th level spell slot.

Harm the casters, break their concentration

The BBEG might have some way to harm the characters without attacking them directly. BBEGs regularly have minions (and if your BBEG doesn't have minions, why not?). The BBEG would order their minions to focus fire on the two casters, attempting to break their concentration.

Until such time that concentration is broken, though, the BBEG is trapped, unable to fight directly, and making saves against sickening radiance. Depending on circumstances, it might be many rounds before the BBEG can escape. This rewards the party's ingenuity.

Teleport out of there

Wall of force does not block teleportation (unlike its higher level counterpart forcecage). If the BBEG has spells such as dimension door or misty step, they can escape the wall of force and continue the fight. (Note that misty step has a range of Self, meaning it is unaffected by wall of force providing total cover.) Misty step is a rather cheap counter, being only a second level spell and a bonus action, but it is also a commonly available one. Dimension door, being only a fourth level slot, also causes the players to have spent more resources than the BBEG, but it is also something which a moderately levelled spellcaster is likely to have.

Spells such as teleport or plane shift can be used by the BBEG to escape the battle entirely. If the goal of the players was to kill the BBEG this is no good for them, but otherwise this leaves the BBEG's lair undefended for the players to loot or demolish or whatever.

Counterspell

If the players are within 60 feet of the BBEG when they enact this plan, the BBEG will want to cast counterspell. Since the player characters are readying an action to cast a spell, it should be obvious to any BBEG worth their salt that they are about to enact some nasty combo and that they should stop that.

However, the BBEG only gets one reaction, so they can only counter one spell. When sickening radiance is cast, they need to decide whether to counter that spell or wait to counter the next spell. If the players had cast the spells individually, the BBEG could have countered them both. But by casting them simultaneously, they guarantee that at least one will hit the BBEG. It also requires the BBEG to use a spell slot (potentially higher than third level, depending how worried they are about the coming combo). This rewards the players' ingenuity without trivialising the encounter.

Dispel Magic

If the BBEG is hit by this combo, they can cast dispel magic on sickening radiance. They are still stuck inside a wall of force, which is not a good place to be, and they may have taken damage from one round of sickening radiance if they failed their Constitution save. The BBEG's escape will be sufficiently delayed that the party might gather into a more advantageous position.

Disintegrate

If the BBEG is high enough level to have disintegrate, then they can use it to get rid of the wall of force. They can then walk out of the sickening radiance, although they'll still have faced at least one round of sickening radiance (depending on their speed). Forcing the BBEG to use their disintegrate on a wall of force and not a player character is a substantial advantage.

Harm the casters, break their concentration

The BBEG might have some way to harm the characters without attacking them directly. BBEGs regularly have minions (and if your BBEG doesn't have minions, why not?). The BBEG would order their minions to focus fire on the two casters, attempting to break their concentration.

However, you specify solo BBEG. If adding minions is not an option, you can instead consider adding lair actions. Lair actions will function as long as the BBEG is alive and can be used to affect the party even if the BBEG can't reach them personally.

Until such time that concentration is broken, though, the BBEG is trapped, unable to fight directly, and making saves against sickening radiance. Depending on circumstances, it might be many rounds before the BBEG can escape. This rewards the party's ingenuity.

Counterspell

If the players are within 60 feet of the BBEG when they enact this plan, the BBEG will want to cast counterspell. Since the player characters are readying an action to cast a spell, it should be obvious to any BBEG worth their salt that they are about to enact some nasty combo and that they should stop that.

However, the BBEG only gets one reaction, so they can only counter one spell. There is the choice of countering wall of force or sickening radiance. They can only stop one spell. If the party did not coordinate their actions, they may have allowed the BBEG a turn between the two casters such that the BBEG could counter both spells. Thus counterspell rewards the players for good teamwork while preventing them from trivialising the encounter.

Note that if Readying a spell is involved, counterspell must be used when the spell is Readied, because that is when casting takes place.

Dispel Magic

If the BBEG is hit by this combo, they can cast dispel magic on sickening radiance. They are still stuck inside a wall of force, which is not a good place to be, and they may have taken damage from one round of sickening radiance if they failed their Constitution save. The BBEG's escape will be sufficiently delayed that the party might gather into a more advantageous position.

Disintegrate

If the BBEG is high enough level to have disintegrate, then they can use it to get rid of the wall of force. They can then walk out of the sickening radiance, although they'll still have faced at least one round of sickening radiance (depending on their speed). Forcing the BBEG to use their disintegrate on a wall of force and not a player character is a substantial advantage.

Harm the casters, break their concentration

The BBEG might have some way to harm the characters without attacking them directly. BBEGs regularly have minions (and if your BBEG doesn't have minions, why not?). The BBEG would order their minions to focus fire on the two casters, attempting to break their concentration.

However, you specify solo BBEG. If adding minions is not an option, you can instead consider adding lair actions. Lair actions will function as long as the BBEG is alive and can be used to affect the party even if the BBEG can't reach them personally.

Until such time that concentration is broken, though, the BBEG is trapped, unable to fight directly, and making saves against sickening radiance. Depending on circumstances, it might be many rounds before the BBEG can escape. This rewards the party's ingenuity.