If you haven’t got an alternate Pokemon for Oichi yet, you will want to do so now- Jigglypuff is nice on defenses, but its offense leaves something to be desired, and Oichi needs a good way to gain more reach. It’s probably a good idea to pick up a Flying-type for her, as those tend to have a high Range and the ability to simply ignore gaps and height differences is very strong. Unfortunately, she has a very poor maximum Link with Zubat, so you’re pretty much stuck getting her a Starly. This isn’t so bad, Starly/Staravia/Staraptor isn’t a poor-quality ‘mon by any means, but the lack of options can be distressing. She should also pick up something else that can handle Rock pokemon in some manner.
You’ll also find that you may be terribly equipped for this fight- Eevee is blunted considerably by Rock and Steel typing, and the weakness of not evolving is starting to show. Piplup is a good option for heading to Terrera in particular, while Sewaddle (if you can evolve it to Swadloon) gives you a slightly risky alternative with a similar range profile and some more interesting later possibilities.
One pokemon that stands to make an excellent showing in Terrera is Wooper/Quagsire. Being both Water and Ground typed, it stands to resist a lot of the potentially-incoming attacks here and can take advantage of one of the main weaknesses in the fortress regardless of whether it is evolved yet or not. Go’s Sandile may also do well, though it’s less of a good option with her low (42%) Link cap using the silly thing.
Regardless, if you’re going to be able to handle the kind of defense and damage output that Tererra’s defenders are capable of, you’re going to want to make sure that a fair chunk of the Pokemon you send in are evolved at least once, with the possible exception of the non-evolving Carnivine (which is kind of risky here with its weakness to Fire attacks).
Once you feel ready to try and handle Tererra’s damage-resistant Pokemon, it’s time to gather up an army- probably in Pugilis- and set out for the fortress. You’ll need to keep on your toes here- the secondary advantage you’re likely to have over most of the Pokemon the warriors there use is Speed, improving the chances of your pokemon evading head-on attacks (and, if you’re lucky, side attacks). This really isn’t much of an advantage to speak of, so have some care with your offensive- lacking defenses can lead you into some real trouble.
Then it’s time to set out.