Pokémon Legends: Arceus does a lot of things differently from the mainline Pokémon games, and that includes completing your Pokédex. Where before the games tasked you with collecting the entire monster compendium to fill out the encyclopedia within the device, Arceus takes place in the past, presumably before Bill invents the internet. At this point in the Pokémon timeline, you can’t just catch one creature and have the device download the species’ entire Wikipedia page. The game requires you to interact, catch, or defeat monsters over and over again so you can learn more about these creatures.
You are, in other words, writing the first Pokédex. Your dex lays out certain conditions for each Pokémon, which, if fulfilled, can level up your research rank. Catching a Pokémon once or defeating them normally will only grant you a limited number of research points, and you have an entire page’s worth of tasks to complete for each and every Pokémon. Plus, you need ten points before a dex entry is considered complete. The thing is, while you’re in the wild, it’s extremely easy to barge into battle or toward a critter without a specific plan. This is a mistake.
Instead, you should press the down button to open your dex, and open it to see the different methods of accumulating points. You can also check a wild Pokémon’s specific dex entry by aiming at them with ZL (left trigger), and then pressing the down button. Even if the enemy is low level, and even if you’ve caught it before, you should check your dex right before interacting with any given monster to ensure that you do so in a way that will help further whatever research goals are either the farthest along, or the easiest to accomplish. Yes, every time.
For the most part, nearly all the requirements are easily doable, but not all of them. Sometimes your dex asks you to capture monsters based on weight, but it’s hard to tell a Pokémon’s weight based on their appearance (unless they’re clearly six feet tall). Being able to fulfil this research point is almost wholly a matter of RNG. Similarly, having to observe an opponent use a specific move during battle is kind of hard. Moves are used entirely at random. So be pleasantly surprised when you get mowed down by a Kricketune’s Slash attack, but don’t count on it. These aren’t the only examples!
Of course, you can absolutely finish your Pokédex by catching and battling ‘em all. But thankfully, Legends doesn’t ask you to.