Riptide does not appear as an enchantment at lower levels.
- Thanks P00R GRENDEL for mentioning this.
From some quick research here: https://minecraft.gamepedia.com/Enchanting/Mechanics#Step_On...
Looks like with 15 bookshelves around the enchantment table. There is a chance you can get riptide 1 with the middle (spend 2 exp levels) but you're greatest chance is of course spending the 3 exp levels. Of course, save before doing the enchantments in case you run out of levels.
Okay so the other solutions posted here were 100% correct but it was taking me way too long to create the enchantment table, get the trident, level up, etc. Then I stumbled upon this solution and it took me about an hour total to get this achievement. I recorded it and made a video :)
Basically load up a tutorial in minecraft xbox one edition, get lots of XP potions, and then flood the zombie spawner (make sure to leave an air pocket for them to spawn).
Then switch to the Minecraft better together version, import the world and let the flooded zombies turn into the aquatic water zombies that have a chance to drop the trident.
Grind it a bit, it shouldn't take too long and then go to the enchanting table. If riptide is not an option, enchant something else for 1 xp bar and retry.
To use the trident with riptide just hold LT while you're in the water.
Hope this helps :) Let me know if there is any changes I can add.