Life of Efflam (also called Euflam, Eflam, Eflamm, etc.)
This life of a Breton saint may have been written as early as 1090 or so, but maybe considerably later. Its presentation of Arthur makes it clear that the author does not know anything about King Arthur.

            Efflam, born a king’s son in Ireland, has decided to leave his wife (without consummating the marriage). He finds a ship and boards it with some holy men as companions. It steers by God’s will and lands them in Brittany, on a beach near a huge cave. They are surprised to see a monster come out of the cave, walking backwards so that its footprints cannot be tracked! 1

[The monster] was careful to avoid Arthur, a very strong man, who in those days was hunting monsters in that part of Brittany. Finally, by God’s will, the wicked  wily was  outsmarted. Arthur, concentrating his whole mind on its activities, tracked it down. While he was investigating the rocks where it might be hiding, he met the holy men on the shore; he expressed admiration at their courage in living in such a horrible solitary wasteland. He asked them carefully who they were and where they had come from; as he questioned them, they told him about the monster and pointed out its cave.  Arthur was delighted by the arrival of the holy men and rejoiced with all his heart at the indication of the monster’s cave. So often he had gone away sad, humiliated as if the monster had beaten him, because he could not [find it to] overcome it. He armed himself therefore with a triple-knotted club; with a shield covered by a lion-skin he defended his brave chest.  Then, manfully, one man fighting for all, he attacked the public enemy.
            Against him, the horrible monster protected itself with its own weapons. Moreover, it attacked Arthur first, indignant that any man should come hunting it, since it had won so many battles against so many groups of people. So, attacking with all its strength, it struck Arthur’s strong shield, which it easily pierced with its sharp claws. No surprise: even the hard stones had been scratched by their points, so that the whole area bore witness by these marks. Arthur, having pulled his body back, remained unwounded, and, angry now, he struck his enemy fiercely. He could have knocked down walls with a blow like that, but the enemy, protected by its hard skin, did not receive a mortal wound. All the same, it felt to the depths of its being the terrible pain of the blow. All day they fought in this way, but the serpent did not get a lethal wound.
            Arthur, realizing that night was coming, put off the battle until the next day. He went back to the holy men, tired from his work and weak from the struggle; his mouth was parched by a powerful thirst, and he asked the men for water. Efflam replied for them all: “My lord, we do not have water, and we, like you, need it; therefore it remains for us to ask our God for it.” Arthur praised the righteous advice of the good man. Bending their knees, they all kneeled on the ground. Efflam, stretching out his whole body, the voice of all his companions, prayed, saying: “God, your immutable power created everything without preexisting matter, and all things serve you according to their properties, and at your order all things change their qualities, so that hard things melt to softness and rough things are made smooth. Proclaim your strength over us, and, as you once ordered water to flow from a rock in the desert for the thirsty people when your servant Moses prayed,2 so, Lord, deign in your great mercy to pour forth water for us.” When he had finished praying, he climbed up on a tall rock, made a little sign of the holy cross, and struck the stone. An abundance of water burst forth into the air.  Giving thanks, the thirsty man [Arthur] drank this water, and then threw himself down at the feet of Saint Efflam, beseeching him with humble prayers to deign to bless him with a laying on of hands, and pray with prayers for him to God. [He needed such prayers,] since he often undertook deeds to free people where he was in doubt of his life. Efflam, that man of God, blessing him, promised him the gift of prayers.

Arthur withdrew, filled with joy, his thirst gone, his strength restored by the divine drink, fortified by the holy blessing. Confident in the strength of the holy man, he left the question of his battle with the monster up to him. The athlete of Christ [Efflam],3 armed with the weapons of faith, 4 ordered the horrible monster to come forth. So that he might strengthen his companions’ faith, he spoke thus with a loud voice: “Lord Jesus Christ, you gave your disciples such power that they might bind and loose whatever they wished,5 and indeed you said of them, ‘they order demons to flee, and they remove serpents.’ 6 Order the departure of this serpent so that this region, freed from such a plague, will be able to give you praise and thanks.” And when they replied “Amen” the monster, which had drawn itself up on a rock, rolled its eyes in all directions and gave forth a cry mixed with a deep miserable sigh, a noise which made far-off places tremble. Then, hanging its head, it brought forth a bloody vomit from its sobbing mouth and nostrils. In witness to this miracle, the rocks in that place seem to shine red as if with recently shed blood. Then, with the holy men watching, it went down to the sea, stretching itself out on the wave, and left, never to return. By these two miracles, God wished to show the power of his servant Efflam at the beginning of his exile: that a man who left his father and mother, and renounced all he possessed in order to follow Him righteously, could have confidence in His grace.

1 The writer may have gotten this idea from the classical story of Hercules and Cacus (Aeneid VIII). Cacus was a monster who stole cattle by making them walk backwards into his cave, so that their footprints pointed away from it. Arthur appears in the story of Efflam with a club and lion-skin, two items associated with Hercules and not found in any other Arthur stories that I know of.

2 Exodus 17:1-7.

3 1 Corinthians 9:25.

4 Ephesians 7:10-17.

5 Matthew 16:19

6 Mark 16:17-18.