Gothita's pretty good considering that it has fairly high special defense for an unevolved pokemon, as you likely won't train it up to level 32 to evolve it. It has psybeam for Tangrowth, which takes on a poison type for its Pulse/pseudo-Mega evolution, and you can give it charge beam for Chatot with the TM from Julia. You might be tempted like I was to swap psybeam for psyshock, which may be better in the long run, but does poorly against Tangrowth, which has a significantly higher defense than special defense. The only problem might be that you get Gothita at a low level by paying that homeless guy in that alley around the starting area of the game (I'm terrible at remembering the names of all of the wards but I think that it's Peridot), so it'll take some time to level up.
For a higher leveled pokemon, you get Litleo at level 15, I think, which will do well against Glaceon and Tangrowth with a few levels. You get it by trading a Bibarel to a guy in a house, in what I think is the upper Peridot ward. Evolving Bidoof doesn't take too long, and Litleo gains experience faster due to being a traded pokemon. Gothita, however, will help against the boss battle not long after this one.