What are the legitimate Sharia boundaries that a Muslim may resort to in order to undo magic, and what is the ruling on going to شيخs who claim to undo magic through "deeds" (الأعمال)?

It is not permissible to undo magic with magic, nor to go to magicians and charlatans, nor to seek treatment by subjugating jinn.

Treatment with ruqyah (Islamic incantation) is permissible, and it is of two types: undoing magic with magic (which is a work of Satan), and an 'an-Nushrah' (a specific method of treatment) using ruqyah, seeking refuge [with Allah], and permissible supplications (and this is permissible).

The most beneficial things to use to remove magic are the two surahs of seeking refuge (Al-Falaq and An-Nas) and Ayah al-Kursiy (the Verse of the Throne). It has also been reported from some of the Salaf (early Muslims) the use of verses that mention the nullification of magic, such as verses from Surah Yunus, Al-A'raf, and Taha.

It is prescribed for the enchanted person and others to fortify themselves with the legislated adhkar (remembrances of Allah), such as the morning and evening adhkar, maintaining wudu (ablution), and reciting Surah Al-Fatihah and Al-Baqarah, for the Qur’an is a healing and mercy for the believers.