- Delete any of Samurott's moves. It doesn't matter which one, only that there's now an empty moveslot.
- Give Samurott a Mirror Herb to hold.
- Find another Pokemon that gets Sacred Sword, such as Gallade. Gender and egg group are irrelevant; even if you have a genderless Iron Leaves that can't make eggs at all, that still works as long as it actually has the move.
- Make sure the Samurott and the other Pokemon with Sacred Sword are both in your party, then start a picnic.
- As soon as you summon all the Pokemon to the picnic, you can immediately end it, and that will be good enough. Samurott will now have Sacred Sword over the empty moveslot (unless it finds another possible egg move earlier in the party), and the Mirror Herb does not get consumed so you can keep reusing it for more egg move transfers. Notably, this process doesn't require getting any actual eggs anymore.
This direct-transfer mechanism, where a move is learned directly by a potential parent instead of being passed down to an egg, also existed in Sword, Shield, Brilliant Diamond, and Shining Pearl. However, the Mirror Herb didn't exist in those games, so it wasn't a requirement. Instead, those games placed the limitation where moves could only transfer if both partners were the same species (and since picnics didn't exist there either, you had to take the two partners to the day care).
