Indra, the Lord of the devas was an exceptionally powerful warrior according to Valmiki Ramayana. He had his fair share of victories and defeats.
A. Victories and acheivements of Indra -
- Indra killed a demoness named Manthara who wanted to destroy the earth -
Oh, Rama, the protector of people, we have heard that Indra once eliminated Manthara, the daughter of Virochana, when she wished to annihilate earth, haven't we.
- Sarga 25, Bala Kanda, Valmiki Ramayana.
2. Indra killed Vishvarupa -
That enraged Hanuma the son of wind-god chopped off with that sharp sword, his three heads furnished with diadems and ear-rings, like Indra the lord of celestials chopped off the heads of Vishvarupa the son of Tvashta.
- Sarga 70, Yuddha kanda, Valmiki Ramayana.
3. Indra killed the asura Puloma -
As soon as possible I wish to eliminate that Ravana with sharp arrows, as has been done by the enemy slayer Indra when he eliminated the insolent father of Sachi Devi, namely Puloma."
- Sarga 39, Kishkindha kanda, Valmiki Ramayana.
4. Indra killed Vrita, Bala and Namuchi -
As with fall of demon Vrita or fall of demon Bala who are eliminated by Indra with his Thunderbolt, or as with the collapse of demon Namuchi who is eliminated by the same Indra just with forth or foam, Khara too is brought down.
- Sarga 30, Aranya Kanda, Valmiki Ramayana.
5. Indra killed Shambara -
Beaten down by me, as Shambara by Indra and Naraka by Vishnu, I will lay down Rama today in battle.
- Sarga 69, Yuddha kanda, Valmiki Ramayana.
6. Indra defeated Kabandha but spared his life -
Having been blessed with long life and thinking, what can Indra do to me?, I attacked him. The thunderbolt having a hundred nodes deployed by Indra pierced my thighs and my head. Entreated, he did not send me to the abode of Yama ( lord of death) and said, 'Let the words of the creator be true'.
- Sarga 71, Aranya Kanda, Valmiki Ramayana.
7. Indra fought evenly with Ravana -
Thereupon Shakra stretched his great bow that gave forth a thunderous noise and, when he drew it, the twanging of that weapon caused the quarters to resound. Then Indra bending that great bow, let fall his flaming arrows like unto fire or the sun on Ravana’s head and, on his side, that powerful titan covered his rival with a hail of arrows loosed from his bow and both showered down missiles on every side so that nothing could be distinguished and the whole firmament was enveloped in darkness.”
- Sarga 28, Uttara Kanda, Valmiki Ramayana.
B. Defeats and retreats of Indra -
- Bali Maharaja defeated Indra and all the devas -
In the meantime, when Vishnu was in his asceticism in this hermitage, the son of King Virochana, namely Bali, has completely conquered all the assemblages of gods together with Indra and his coterie called Wind-gods, and ruling that kingdom where that hermitage is there, he has become renowned in all the three worlds for his magnanimity.
- Sarga 29, Bala Kanda, Valmiki Ramayana.
2. Indrajit defeated Indra by using the Tamasi Maya obtained from Lord Shiva -
Standing in his chariot, Ravani, who was transported with rage, penetrated into the ranks of the redoubtable Celestial Host and routed the army by resorting to the power of illusion that had been conferred on him by Pashupati. Then, leaving the Gods aside, he rushed on Shakra himself, and the highly energetic Mahendra did not observe the son of his adversary. The Gods, however, whose strength was immeasurable, severed Ravani’s armour, even wounding him, but he remained unperturbed and with his excellent shafts pierced Matali who was advancing towards him, covering Mahendra afresh with a hail of missiles. Thereupon Shakra descended from his chariot dismissing his charioteer, and mounting Airavata, he pursued Ravani, who had made himself invisible through his magic power, but he, springing into the sky, assailed him with arrows. Seeing that Indra was exhausted, Ravani bound him by his magic and led him to the side where his own army was.
- Chapter 29, Uttara Kanda, Valmiki Ramayana.
3. Kumbhakarana made Indra flee to Brahma's world -
Enraged, Mahendra struck Kumbhakarna with his mighty thunderbolt and, at the impact of Indra’s weapon, that giant reeled howling with terror. When the cries of the titan were heard, the fear of those beings increased, and in his anger, the mighty Kumbhakarna tore out one of Airavata’s tusks in order to pierce Vasava’s breast and, under the blows of that monster, Indra appeared like a flame. Seeing this, the Devas, Brahmarishis and Danavas were thunderstruck and Shakra with those Celestials went to Svayambhu’s abode and informed Prajapati of Kumbhakarna’s wickedness.
- Sarga 61, Yuddha kanda, Valmiki Ramayana.
4. Atikaya neutralized Indra's vajra with his arrows -
He is Atikaya, who paralyzed the thunderbolt of the intelligent Indra with his arrows. He struck down the noose of Varuna, the seizer of transgressor.
- Sarga 71, Yuddha kanda, Valmiki Ramayana.
5. All the devas ( which includes Indra) fled in terror when Maharaksha invoked the spear of Shiva -
Deprived of his car, Maharaksha, that prowler of the night, stood on the ground and, armed with his spear, he shone like Fire at the dissolution of the worlds; and he was irresistible with his great lance, a gift from Shiva, that glittered in the air like unto the weapon of destruction. Beholding that great spear that emitted flames, the Gods, struck with terror fled on all sides whilst that ranger of the night, lifting it up, hurled it with fury against the magnanimous Raghava.
- Sarga 79, Yuddha kanda, Valmiki Ramayana.
Overall despite his many defeats, Indra is still an exceptionally powerful warrior capable of striking down even the most powerful Daityas.