First off, is it safe to assume that the croagunk is already used to living without one of its eyes? If no, I’ll cover that in a moment, but first, allow me to answer this as if your croagunk is already adjusted.
The biggest thing you need to know when dealing with a pokémon that’s disabled in some way is understanding what that disability actually means. In this example, you’ll need to know how the eyes work together in order to understand how best to work with your croagunk.
You see, when you lose an eye, the biggest thing is that your depth perception is affected. This in turn means your ability to judge distances will be affected, which, for a frog-like fighting-type pokémon, is a pretty big deal. While your croagunk may have had time to learn how to work around this and adjust accordingly, it’s a good idea to keep this in mind when giving orders. Get to know your croagunk’s battle style. Observe the exact things it does to prepare for a strike, and incorporate your observations into the orders you give it, even in the heat of the moment.
Then, of course, there’s also the fairly obvious point that your croagunk will have a blind spot. Again, this may be something it’s figured out how to compensate for, but be prepared to warn your croagunk if an opponent or other obstacle comes in from its blind side. Be particularly alert and train yourself to give orders quickly—perhaps even instinctually.
If, however, your croagunk has only recently lost its eye, you’ll first want to take it to a Nurse Joy to have it examined. Of course, you’ll want to do this either way, but a recent loss may need to be checked and monitored for potential infection.
After that, you’ll need to begin training. It may be slow going, especially given that your croagunk isn’t used to you, but have patience and be gentle. If you live in a town or city, you may also have access to therapists at your local pokémon center who may be able to work with you and croagunk. It certainly doesn’t hurt to ask. If not, consider asking Nurse Joy herself if she can offer therapy.
Outside of the pokémon center, don’t be overbearing when helping your croagunk. The last thing you want to do is rob it of its independence. Instead, teach it (how to punch, grasp, or leap) only until it starts to get the idea, then let it do it on its own. Be there, of course, if your croagunk is really struggling to adjust its movements to its eyesight, but wait until it signals for you to help. This can take different forms, depending on your croagunk’s disposition. If it looks like it’s getting angry, reassure it first that it’s doing the best that it can, then nudge its target a little closer to it until it can reach for it itself.
This, of course, isn’t the only possible signal. It might also grasp at the target, then look at you. If it does this, guide its hand to the target and then encourage it to follow through itself. Reward it every time it successfully reaches for its target on its own so it begins to learn how to calibrate itself in space without your cues, so to speak.
Also, concerning the blind spot, avoid approaching your croagunk from the side where it’s lost its eye. This is true even if it’s used to seeing with only one eye, but if it’s new to partial blindness, you don’t want to startle it. If you must, verbally let your croagunk know that you’re approaching so it can turn if it needs to. If it seems upset that it can’t see out of one eye, I would recommend a training session tailored towards helping it develop its spacial perception, as noted above, in a battle scenario. Don’t have it spar, of course, but rather have it train against stationary objects. The battling will hopefully boost its confidence enough that it will regrow its sense of self and inner strength. Alternatively, you can achieve the same effect with basic training exercises for day-to-day life or even for whatever its interests are. For example, if it’s not much of a battler and more of an artistic croagunk, consider setting up an art station in your home and having it create its own art. Any activity that relies on visual skills can be used for retraining, so pay attention to what catches your croagunk’s attention.
Best of luck!