Legacy controls with keyboard+mouse have a few more movement options than with a controller. To maintain auto-attacks, you can either input+hold both mouse buttons and S, then use W+A to strafe while swinging the camera with the mouse, or reduce the number of inputs while simultaneously reducing available keybinds by simply binding strafe keys instead.

A legacy controls bard can severely reduce time facing away from the boss while backing up by abusing the "snapping" feature of clicking both mouse buttons: your character will instantly turn to face forward, so you can sort of "bunny hop" backwards by inputting S->Space->Left+Right Click in quick succession to maintain momentum through jumping and turn in the air. I use this technique to try to maintain auto-attacks during "clock" Heavensfall in Savage Turn 9 if I have to move directly away from Nael and I'm confident that the situation is such that an improper input won't kill me.