Albert Barnes Commentary 1 Samuel 29:6

"Then Achish called David, and said unto him, As Jehovah liveth, thou hast been upright, and thy going out and thy coming in with me in the host is good in my sight; for I have not found evil in thee since the day of thy coming unto me unto this day: nevertheless the lords favor thee not." — 1 Samuel 29:6 (ASV)

As the Lord liveth — It seems strange for a Philistine to swear by Yahweh. However, it is likely that the general meaning of this and similar speeches has been preserved, rather than the exact wording.