One wanted to become fierce or fearless, according to scriptures/āchāryas who one should worship? And what is the method? Better if worshipping include reading only Stotra or hymns.
One can recite the so called Abhaya Mantras from the Vedas. The name of the Mantras itself suggests that those are to give freedom from fear.
abhayam nah karati antariksham
abhayam dyAvAprithivi ubhe ime
abhayam pashchAdbhayam purastAt uttarAt adharAdabhayam no astu ||
May the mid-region free us from all fear, both heaven and earth make us fearless; Let there be no fear from behind, no fear from in front, no fear from the north or the south. AV 19.15.5
abhayam mitrAdabhayam amitrAt,
abhayam gyAtAdbhayam parokshAt
abhayam naktamabhayam divA
nah sarvA AshA mama mitram bhavantu ||
Let there be no fear from friend, no fear from foe, no fear from the known, no fear from what lies before us unknown; Let there be no fear for us from night, no fear from day; Let all the quarters be our friends. AV 19.15.6
(AV here stands for Atharva Veda).
As regards Stotrams, then reciting those dedicated to Goddess DurgA will make one completely fearless.
For example the DurgA Sapta Shloki explicitly mentions that:
दुर्गे स्मृता हरसि भीतिमशेषजन्तोः
स्वस्थैः स्मृता मतिमतीव शुभां ददासि ।
दारिद्रयदुःखभयहारिणि का त्वदन्या
सर्वोपकारकरणाय सदार्द्र चित्ता ॥२॥
Durge Smrtaa Harasi Bhiitim-Ashessa-Jantoh
Svasthaih Smrtaa Matim-Atiiva Shubhaam Dadaasi |
Daaridraya-Duhkha-Bhaya-Haarinni Kaa Tvad-Anyaa
Sarvo[a-U]pakaara-Karannaaya Sadaa-[Aa]rdra Cittaa ||2||
2.1: (Salutations to You, O Jagadamba) O Devi Durga, whoever Remembers You with Devotion, You Remove the very Root of their Fear. 2.2: Whoever Meditates on You as being present within their own Hearts, with Extreme Devotion, You Bestow (i.e. Reveal to them) Your Auspicious Nature (which is beyond description), 2.3: O Mother, Apart from You, Who Else can Destroy Poverty, Sorrow and Fear from our Lives? (which appears to be a never-ending cycle), 2.4: Your Heart is Always Full of Compassion to Render All sorts of Help to Your Devotees.
And, besides this Stotra, i know many other Stotras of Goddess DurgA too, which mention in their Phalasrutis, a complete eradication of fear for those who recite those. These Stotras are from the Tantras like KubjikA Tantram etc. But i can't find these Stotras anywhere online.
For Shiva Stotras, one can recite the Shiva Raksha Stotram composed by Yajnavalkya Rishi. The Phalasruti of this Stotra mentions:
Abhayam kara namedham kavacham Parvathi pathe,
Bhakthya bibarthee ya kande, thasya vasyam jagat thrayam,
Imam Narayana swapne Shiva Raksham yadha disath,
Prathar uthaya yogeendro, Yagna valya sthadha likath. 10
This armour of the names of the consort of Parvathi, Would remove fears and provide protection, To the devotees who sing these often, And the lord of the three worlds would be within his hold, For this protection of Lord Shiva was revealed, By Lord Vishnu in the dream to Yagna Valkya, Who wrote it, as he was told, as soon as he woke up in the morning.
Choose any one of the following choices :
1) Hanuman (Bajrang bali)
text= Hanuman chalisa
mantra= Bagalamukhi mahamantra
Bagalamukhi Devi smashes the devotee's misconceptions and delusions (or the devotee's enemies)
3) mahakaleshwar and mahakali form of shiva and parvati
mantra = उग्रं वीरं महाविष्णुं ज्वलन्तं सर्वतोमुखं । नृसिंहं भीषणं भद्रं मृत्युमृत्युं नमाम्यहं ।।
list of text is available here https://en.wikipedia.org/wiki/Narasimha#Prayers
In his forms, he has got three eyes, like shiva.
Conclusion : If you are already a devout vaishnav, then Narsimha is a good choice. If you are already a devout shakt only then bagalamukhi or mahakali is a good choice. If you are shaiva or shakt or a mix, then mahakal+mahakali is a good choice. Hanuman ji is good for every sect, but then use his name as "bajrang bali" - that will give more fearlessness.
I am answering only this part:
One wanted to become fierce or fearless, according to scriptures/āchāryas who one should worship?
According to Devi Bhagavatam, Book 8, Chapter IX, by worshipping Lord Nrisimha one can become fearless.
Prahlâda spoke :-- I bow down to Thee, the Bhagavân Nrisingha Deva. Thou art the Light of all Lights. Thy big teeth are like thunderbolts. Let Thee manifest in Thy most terrible form. Let Thee destroy the desires of the people to do Karma and let Thou devour the great Ignorance (Ajñâna) the Moha (delusion) of the people. Thou art the receptacle of the Sattva, Raja and Tamo Gunas. Let myself be always free from any fear by Thy Grace.
So leave, at once, the home that is the source of Birth and Death and leave Trisnâ (thirst, desire), clinging to life, low-spiritedness, name, and fame, egoism, shame, fear, poverty and loss of one's honour and worship the Lotus-Feet of the Bhagavân Nrisingha Deva and be entirely fearless.
My Lord Nṛsiṁhadeva, please, therefore, cease Your anger now that my father, the great demon Hiraṇyakaśipu, has been killed. Since even saintly persons take pleasure in the killing of a scorpion or a snake, all the worlds have achieved great satisfaction because of the death of this demon. Now they are confident of their happiness, and they will always remember Your auspicious incarnation in order to be free from fear.
Lord Shiva is also worshipped to get the freedom from fear. It is mentioned in Shiva Maha Purana, Part 1, Chapter 17 of Vidyeshwara Samhita translated by J.L. Shastri.
38-42. The five-syllabled mantra shall be repeated five hundred thousand times, all the time remembering the various aspects of Lord Siva who is seated in the lotus pose. He is the bestower of all auspiciousness. He has the crescent moon for his coronet. He has given shelter to Gangā in His matted hair. With Sakti seated on His left thigh, He shines with His great concourse of attendants around Him. He bears the moon (on his forehead). He shows the gestures of bestowing boons and offering freedom from fear. He is the cause of perpetual blessing. He is Sadasiva. He shall be mentally worshipped at first or as stationed in the heart or in the solar zone. While performing the Japa of the five-syllabled mantra he shall sit facing the cast. All his actions shall be pure. In the inorning of the Caturdaši day of the dark half, after finishing the daily rites he shall sit in a clean beautiful place. He shall control his mind and senses. He shall repeat the five-syllabled mantra twelve thousand times in this way.
Meditation on Om for Fearlessness .
Chandogya Upanishad tells us that the Gods became fearless by meditating and entering in syllable "Om-ॐ" . One becomes fearless when he masters the Rig ,Yajur and Sama Veda and utters the "OM" swara by completly getting absorbed in it.
यदा वा ऋचमाप्नोत्योमित्येवातिस्वरत्येव सामैवं यजुरेष उ स्वरो
यदेतक्षरमेतद्म्रुतमभयं तत्प्रविश्य देवा अमृता अभया अभवन् || 4||
When one gets (learns) the Rik , he loudly pronounces "OM" ; the same with the Sama and yajus. And this Swara (accent) is the syllable , the immortal and fearless. Having entered this Gods became immortal and fearless.
So in order to develop fearlessness or for freedom from fear of any kind one can meditate on Swara "OM" i.e. Nada-Brahman.
And Here the translation of Mantra in Hindi.
Lord Siva or Rudra represents the fierce aspect of Brahman. We can worship him to get these qualities like him from him through stotras and mantras.
For those who can not recite vedic mantras , they can worship Lord Shiva for the same purpose here is Shiva-Kavacha which asks the lord to make us fearless.
सर्वत्र मां रक्षतु विश्वमूर्ति-र्ज्योतिर्मयानन्दघनश्चिदात्मा ।
अणोरणीयानुरुशक्तिरेकः स ईश्वरः पातु भयादशेषात् ॥ 8॥
Sarvathra maam rakshathu viswa moorthir, Jyothir mayananda ghanaschidathma,
Anoraniyanuru shathireka, Sa iswara pathu bhayad aseshath.
May He render me free from all fears in all places, who exists in all forms, who is all-blissful, who is smaller than the smallest and is possessed of Mighty Power.
So one can worship Lord Shiva or Rudra for fearlessness. One can recite this mantra from the Kavacha with guidance from an expert.