You obtain him exclusively through the gacha system (a.k.a. Wishes). If you get lucky enough on any of the banners (as in 0.6% to get any 5-star champion normally), then you’ll get him, probably. Even the pity rules won’t give you a sure chance of getting Diluc, because again pity only assures you of a 5-star champion OR item.
You can unlock him temporarily as part of one of the in-game quests, but otherwise he is completely inaccessible.