Lord Vishnu is generally considered a cheerful and pleasant god. But, Lord Vishnu takes his fierce form when his devotees are harmed in any manner. In life, we may undergo lots of misery and have many enemies. There may be many people that prevent us from leading a happy life. In such situations, you may use the below mantra to call upon Lord Vishnu with devotion for his protection.
Om shreem krishnaya shreem shreem shreem govindaya gopalaya goloka sundaraya sathyaya nithyaya paramathmane paraya vykhanasaya vyrajamoorthaye meghathmane shreem narasimhavapushe namah
Chant the above mantra 91 times each in the morning and the evening. Start the chanting on a thursday morning and continue for 27 days. It is advisable to light a lamp and then sit in front of it while chanting the mantra. You may shower flowers onto a photo or idol of lord Vishnu if possible, while chanting the mantra.