Ashwathama was not a baby warrior, he was an amazingly skilled warrior. Even krishna appreciated his skill and called him superior than Arjuna. Krishna even humiliated Arjuna by giving example of Ashwathama's Superb archery skills. He was one of the few people on earth to have knowledge of Bhramastra. He also had the knowledge of entering and exitting Chakravyuhs without getting killed (like abhimanyu lol) Here is a narrative which describes defeat and humiliation of poor Arjun.
the son of Drona quickly proceeded against Arjuna, that foremost of victorious warriors. Indeed, Drona's son rushed, shaking his formidable bow, and taking with him many terrible arrows like the Sun himself appearing with his own rays. With mouth wide open from rage and with the desire to retaliate, and with red eyes, the mighty Ashvatthama looked formidable like death himself, armed with his mace and filled with wrath as at the end of the Yuga. He then shot showers of fierce shafts. With those shafts sped by him, he began to rout the Pandava army. As soon as he beheld him of Dasharha's race (Keshava) on the car, O king, he once more sped at him, and repeated showers of fierce shafts. With those falling shafts, O monarch, sped by Drona's son, both Krishna and Dhananjaya were completely shrouded on the car. Then the valiant Ashvatthama, with hundreds of keen arrows, stupefied both Madhava and the son of Pandu in that battle. Beholding those two protectors of all mobile and immobile creatures thus covered with arrows, the universe of mobile and immobile beings uttered cries of "Oh!" and "Alas!" Crowds of Siddhas and Charanas began to repair to that spot from every side, mentally uttering this prayer, viz., "Let good be to all the worlds." Never before, O king, did I see prowess like that of Drona's son in that battle while he was engaged in shrouding the two Krishnas with shafts. The sound of Ashvatthama's bow, inspiring foes with terror, was repeatedly heard by us in that battle, O king, to resemble that of a roaring lion. While careering in that battle and striking right and left the string of his bow looked beautiful like flashes of lightning in the midst of a mass of clouds. Though endued with great firmness and lightness of hand the son of Pandu, for all that, beholding the son of Drona then, became greatly stupefied. Indeed, Arjuna then regarded his own prowess to be destroyed by his high-souled assailant. The form of Ashvatthama became such in that battle that men could with difficulty gaze at it. During the progress of that dreadful battle between Drona's son and the Pandava, during that time when the mighty son of Drona, O monarch, thus prevailed over his antagonist and the son of Kunti lost his energy, Krishna became filled with rage. Inspired with wrath he drew deep breaths, O king, and seemed to burn with his eyes both Ashvatthama and Phalguna as he looked at them repeatedly. Filled with rage, Krishna addressed Partha in an affectionate tone, saying, "This, O Partha, that I behold in battle regarding thee, is exceedingly strange, since Drona's son, O Partha, surpasseth thee today! Hast thou not now the energy and the might of thy arms thou hadst before? Hast thou not that Gandiva still in thy hands, and dost thou not stay on thy car now? Are not thy two arms sound? Hath thy fist suffered any hurt? Why is it then that I see the son of Drona prevail over thee in battle? Do not, O Partha, spare thy assailant, regarding him as the son of thy preceptor, O bull of Bharata's race."
See how arjuna was humiliated by Krishna. Krishna clearly said, don't you have the energy in your hands that you had before? isn't Gandiva still in your hands? aren't you on your chariot now ? Are your two arms not strong (sound)? Is your fist hurt?
Arjun fans claim that Arjun is the best, which is clearly bullshit as shown by the above narrative.