Narasimha-Sadhana is to be resorted to when various Mars afflictions are approaching; for the protection from black magic, pishachas, bhutas and etc; against snakes, Sharpa-doshas particularly; evil dreams; poverty and etc.
If you are interested in removing all the Navagraha Doshas see the Upadesha from The Verse 28 of "Shree Narasimha-Kavacha":
"तिलकं विन्यसेद्यस्तु तस्य ग्रहभयं हरेत् | त्रिवारं जपमानस्तु दत्तं वार्यभिमन्त्र्य च - Having marked ones body with Tilaka, taking Achamana with water, and reciting this Mantra (Kavacha) 3 times, one will find that the fear of all inauspicious planets is removed."
From Bhakti point of view, Narasimha Sadhana is aimed at the removing various Bhakti-obstacles, such as bad health (at reasonable rate, of course, if you are in the terminal stage of cancer, then Mantras will be of no help, because some part of prarabdha karma is impossible to eradicate), tamasik aptitude and etc.
CRUCIAL 01 - Mantras + Kavacha: I do believe that Mantras don't work at all without Kavachas.
For example, if you recite The Ashtakshara Mantra of Bhagavan Shree Narayana, you HAVE TO recite "Shree Narayana Kavacha" every day. The same rule applies to The Mantras of Bhagavan Shree Narasimha.
CRUCIAL 02 - No tamas: Never use Narasimha-Mantras for