In Mahabharat Yakhsha Parva, when the pandavas try to drink water from a pond, a Yakhsha appears. He warns the pandavas not to drink water from that pond before answering his questions. But the 4 pandavas (except Yudhisthira) drank and they died instantly. Was that Yaksha so much mighty that even warriors like Arjuna and Bhima could not at least fight him (if not defeat).
That was not an Yaksha. It was Yama in the disguise of a yaksha -
'O child, I am even your father, the Lord of justice, possessed of great prowess! Know, bull of the Bharata race, that I came hither desirous of beholding you! Fame, truth, self-restraint, purity, candour, modesty, steadiness, charity, austerities and Brahmacarya, these are my body! And abstention from injury, impartiality, peace, penances, sanctity, and freedom from malice are the doors (through which I am accessible).'
And even then, it seemed that Pandavas died because water was poisoned. Yama didn't defeat them in any actual battle. Remember that Arjuna earlier had defeated all the gods including Yama at Khandava forest -
But Arjuna and Krishna, fearless and invincible in battle, beholding Sakra and the other celestials prepared for fight, calmly waited, bows in hands. Skilled in battle, those heroes in wrath assailed the advancing host of celestials with their own thunderlike arrows. The celestials repeatedly routed by Krishna and Arjuna, at last left the field of battle for fear and sought the protection of Indra.