Systems and methods for forming a lesion on an endocardial tissue of a patient's heart involve placing an ablation assembly inside of the heart and adjacent to the endocardial tissue, and placing a guiding assembly outside of the heart. An ablation assembly includes an ablation element and a first attraction element, and a guiding assembly includes a second attraction element. First and second attraction elements can be attracted via magnetism. Techniques involve forming an ablation on the cardiac tissue of a patient's heart with an ablation element of the ablation assembly. Optionally, techniques may include moving the second attraction element of the guiding assembly relative to the patient's heart, so as to effect a corresponding movement of the ablation element of the ablation assembly.