What is the way to be saved from groundless misgivings (waswas) that have no remedy, especially the misgivings regarding the takbir in prayer, and is ignoring them the only treatment, despite its extreme difficulty?

The cure for obsessive thoughts is to ignore them and turn away from them. This may seem difficult at first, but whoever seeks Allah's help and strives against their self will recover. Many have been afflicted by this and then Allah cured them through following this remedy. Imagining an inability to get rid of obsessive thoughts is an illusion from Satan. With sincere will, determination, and struggle, one can overcome them. In addition, strive in supplication and adhere to remembrance of Allah. There is no harm in consulting trusted doctors for help with physical medications.