Out of game

Double check the character is built within the rules

The PC as you describe is far stronger than a standard Nymph. More than 5 levels of fighter strong. You should check that you have followed the rules correctly and if you find an error you can use this as a basis for re-building the character without breaking anything dramatically. Admit you made an error allowing it and take away abilities she should never have had in the first place.

Some areas I have noticed just from your description (there may be more if we see the full character sheet):

• You allowed +4 to any stat from the monster advancement rules these rules are not intended to work with the monstrous pcs rules. If you advance a monster under these rules before adding class levels you should calculate the new CR as a base to add levels to.
• Nymphs do not have natural armour and she is therefore not eligble for the Improved Natural Armor feat.

This will go some way to address the issue but may not solve it entirely.

Talk to the player(s)

You mention that some of the players aren't enjoying it, it's possible that she isn't enjoying the session either. You need to talk to her to she where she stands.

If she doesn't want to change you can discuss with the entire group what a good solution would be. Potentially you can buff all the other characters to level out the party and then just crank the combat difficulty right up to compensate.

In Game

Above I present how I would approach the situation. However, you have indicated you would prefer not to talk to her outside the game, or rebuild the character. If you won't consider out of game options it will be difficult to make major changes but there are a few things you can do.

Consider the advice given in this question about de-powering a high AC character without hurting the party.

Impose Druid limitations

Another potential fix would be to take away some of her Nymph abilities for wearing armour. Nymph's are considered Druid Spellcasters:

A nymph casts spells as a 7th-level druid, but cannot swap out prepared spells to cast summon spells.

So it wouldn't be unreasonable that some abilities don't work properly while wearing armour. You could potentially impose other druid limitations on her as well at your discretion.

Spread Rumours

She is the most persuasive and beautiful creature to have ever walked the earth. NPCs will be aware of her. Powerful NPCs may take precautions to avoid being convinced by her, or will always start as hostile (increasing the DC). Others may seek her out, professing their love or whatever other motives they may have for wanting a Nymph as an allies (or slave).

Out of game

Double check the character is built within the rules

The PC as you describe is far stronger than a standard Nymph. More than 5 levels of fighter strong. You should check that you have followed the rules correctly and if you find an error you can use this as a basis for re-building the character without breaking anything dramatically. Admit you made an error allowing it and take away abilities she should never have had in the first place.

Some areas I have noticed just from your description (there may be more if we see the full character sheet):

• You allowed +4 to any stat from the monster advancement rules these rules are not intended to work with the monstrous pcs rules. If you advance a monster under these rules before adding class levels you should calculate the new CR as a base to add levels to.
• Nymphs do not have natural armour and she is therefore not eligble for the Improved Natural Armor feat.

This will go some way to address the issue but may not solve it entirely.

Talk to the player(s)

You mention that some of the players aren't enjoying it, it's possible that she isn't enjoying the session either. You need to talk to her to she where she stands.

If she doesn't want to change you can discuss with the entire group what a good solution would be. Potentially you can buff all the other characters to level out the party and then just crank the combat difficulty right up to compensate.

In Game

Above I present how I would approach the situation. However, you have indicated you would prefer not to talk to her outside the game, or rebuild the character. If you won't consider out of game options it will be difficult to make major changes but there are a few things you can do.

Consider the advice given in this question about de-powering a high AC character without hurting the party.

Impose Druid limitations

Another potential fix would be to take away some of her Nymph abilities for wearing armour. Nymph's are considered Druid Spellcasters:

A nymph casts spells as a 7th-level druid, but cannot swap out prepared spells to cast summon spells.

So it wouldn't be unreasonable that some abilities don't work properly while wearing armour. You could potentially impose other druid limitations on her as well at your discretion.

Spread Rumours

She is the most persuasive and beautiful creature to have ever walked the earth. NPCs will be aware of her. Powerful NPCs may take precautions to avoid being convinced by her, or will always start as hostile (increasing the DC). Others may seek her out, professing their love or whatever other motives they may have for wanting a Nymph as an ally (or slave).

Out of game

Double check the character is built within the rules

The PC as you describe is far stronger than a standard Nymph. More than 5 levels of fighter strong. You should check that you have followed the rules correctly and if you find an error you can use this as a basis for re-building the character without breaking anything dramatically. Admit you made an error allowing it and take away abilities she should never have had in the first place.

Some areas I have noticed just from your description (there may be more if we see the full character sheet):

• You allowed +4 to any stat from the monster advancement rules these rules are not intended to work with the monstrous pcs rules. If you advance a monster under these rules before adding class levels you should calculate the new CR as a base to add levels to.
• Nymphs do not have natural armour and she is therefore not eligble for the Improved Natural Armor feat.

This will go some way to address the issue but may not solve it entirely.

Impose Druid limitations

Another potential fix would be to take away some of her Nymph abilities for wearing armour. Nymph's are considered Druid Spellcasters:

A nymph casts spells as a 7th-level druid, but cannot swap out prepared spells to cast summon spells.

So it wouldn't be unreasonable that some abilities don't work properly while wearing armour. You could potentially impose other druid limitations on her as well at your discretion.

In Game

Above I present how I would approach the situation. However, you have indicated you would prefer not to talk to her outside the game, or rebuild the character. If you won't consider out of game options it will be difficult to make major changes but there are a few things you can do.

Consider the advice given in this question about de-powering a high AC character without hurting the party.

Spread Rumours

She is the most persuasive and beautiful creature to have ever walked the earth. NPCs will be aware of her. Powerful NPCs may take precautions to avoid being convinced by her, or will always start as hostile (increasing the DC). Others may seek her out, professing their love or whatever other motives they may have for wanting a Nymph as an allies (or slave).

Out of game

Double check the character is built within the rules

The PC as you describe is far stronger than a standard Nymph. More than 5 levels of fighter strong. You should check that you have followed the rules correctly and if you find an error you can use this as a basis for re-building the character without breaking anything dramatically. Admit you made an error allowing it and take away abilities she should never have had in the first place.

Some areas I have noticed just from your description (there may be more if we see the full character sheet):

• You allowed +4 to any stat from the monster advancement rules these rules are not intended to work with the monstrous pcs rules. If you advance a monster under these rules before adding class levels you should calculate the new CR as a base to add levels to.
• Nymphs do not have natural armour and she is therefore not eligble for the Improved Natural Armor feat.

This will go some way to address the issue but may not solve it entirely.

Talk to the player(s)

You mention that some of the players aren't enjoying it, it's possible that she isn't enjoying the session either. You need to talk to her to she where she stands.

If she doesn't want to change you can discuss with the entire group what a good solution would be. Potentially you can buff all the other characters to level out the party and then just crank the combat difficulty right up to compensate.

In Game

Above I present how I would approach the situation. However, you have indicated you would prefer not to talk to her outside the game, or rebuild the character. If you won't consider out of game options it will be difficult to make major changes but there are a few things you can do.

Consider the advice given in this question about de-powering a high AC character without hurting the party.

Impose Druid limitations

Another potential fix would be to take away some of her Nymph abilities for wearing armour. Nymph's are considered Druid Spellcasters:

A nymph casts spells as a 7th-level druid, but cannot swap out prepared spells to cast summon spells.

So it wouldn't be unreasonable that some abilities don't work properly while wearing armour. You could potentially impose other druid limitations on her as well at your discretion.

Spread Rumours

She is the most persuasive and beautiful creature to have ever walked the earth. NPCs will be aware of her. Powerful NPCs may take precautions to avoid being convinced by her, or will always start as hostile (increasing the DC). Others may seek her out, professing their love or whatever other motives they may have for wanting a Nymph as an allies (or slave).