Aromatisse: B tier, possibly A
Availability: Route 7, as a Spritzee
Stats: 101 / 72 / 72 / 99 / 89 / 29
A very passable 101/72/89 bulk coupled with 99 Special Attack; not only can it tank attacks, but also can dish out some damage as well. The only real downside is its paltry speed ticking at 29, even slower than Slowbro. However with her movepool, it more than makes up for the damage it takes in the form of a good HP draining attack (Draining Kiss).
Typing:
It lends useful resistances to Fighting, Dark, Bug, and boasts an immunity to Dragon. Its weaknesses, Poison and Steel are quite uncommon throughout the game and thus, isn't as big of a problem.
Movepool:
She gains useful moves by level-up alone in the form of Calm Mind, Draining Kiss, and Aromatherapy which is all achieved in late 10's to mid 20's. With this, she gains a boosting buff to her decent Special Attack and when used in tandem with Draining Kiss, creates an effective damage-dealing recovery move. Other than those though, I think there aren't any moves worth mentioning aside from Moonblast, Psychic, and Reflect which she gets quite late already. Now, speaking of her TM moves, it isn't too shabby as well. Some of them there are Charge Beam, Toxic, Energy Ball, Thunderbolt, Psyshock, Flash Cannon, and Light Screen. Seeing this, Charge Beam, Thunderbolt, and Toxic might see some use, but it is generally better to stick with the Calm Mind set to get the best out of Aromatisse. One thing that is important to note is that Draining Kiss has only 10 PP so choosing a coverage move or Moonblast is advisable. So by now, her set looks like in the lines of Calm Mind l Draining Kiss l Filler move l Aromatherapy / Filler move.
Major Battles:
A basic strategy to go for against the Gym Leaders and the Elite Four is to boost up with Calm Mind, and work from there using Draining Kiss to recover back any damage received while setting up. Simple, simple.
Grant: Werks it out.
Korina: Annihilates.
Ramos: Werks it out.
Clemont: Werks it out. Magneton however, is a little B.
Valerie: Werks it out.
Olympia: Werks it out.
Wulfric: Werks it out. However, beware of Wulfric's Avalugg as it packs Gyro Ball. Aside from that, she may also falter into repeated Physical beatings.
Wikstrom: Lawd, have mercy.
Malva: Subpar.
Drasna: Dragalge is a problem, but once the threat is taken care of, you are now free toboost up and spam Draining Kiss.
Siebold: Werks it out.
Diantha: Werks it out.
Team Flare: Aromatisse lulls their Dark and Fighting types to sleep, but does get in trouble with the Poison types here and there.
As you can see, Aromatisse doesn't really have that many good type matchups against the Gyms and the Elite Four, resisting and attacking mostly in neutrality. However with her bulk combined with Calm Mind and Draining Kiss, she can muscle through them, set-up and potentially sweep.
Additional Comments: Overall, Aromatisse is very interesting. Most of the times she can handle herself in solo, acting as a tank and a part-time standalone sweeper without that many support needed; a very self-sufficient Pokemon if you will.
And its adorable, too.