Level 50 Statistics (see level 5, 50, 100)
Roselia has a lot in common with Sunflora in NU with the added benefits of Spikes and Aromatherapy while sacrificing some staying power. It is a popular choice on NU teams, as it makes a good counter to Hidden Power Grass Plusle and can scare off Whiscash and Relicanth. Just be careful switching in as Roselia does not like any physical hits at all.
The point of this set is to get Spikes on the field. Period. Roselia can make for a decent special defender in the Never Used metagame and can use that to set up Spikes. Delibird is the only Rapid Spinner allowed in NU, so the advantages to getting Spikes in are obvious for a defensive team (yes, it is possible to create a defensive NU team). Grass is a needed offensive type in NU for all of the Rock and Ground threats, especially Whiscash and Relicanth. The last move is personal preference. Synthesis is probably the best bet so you can survive to get all three layers down but that need can be quelled a bit if you play Leech Seed well. Aromatherapy is an option but not typically needed in NU. Substitute goes well with Leech Seed and can ease prediction on when to Spike. If you're into that, you may want to look into the next set. Poison Point is an option with Aromatherapy, otherwise you'll likely prefer Natural Cure.
Roselia has a surprising Special Attack stat, especially for NU. This set should look very familiar if you have played with Sceptile at all. Leech Seed if you can take the incoming hit or on a switch and Substitute if you're faster. Continue to Substitute until your opponent is within KO range of one of your moves. Magical Leaf is here because you'll want Hidden Power Ice for Murkrow, Pidgeot and other Fliers. Hidden Power Fire can be used for Mawile though, and still hits other Grass types.
GrassWhistle, Stun Spore, and Toxic are available for status. Giga Drain if you prefer recovery with your offense. Growth is available to up your Special Attack but generally not recommended as you'll have neither the time nor Speed to use it.
On the Spiker set you can adjust your Defense if you want to survive something specific, but Special Defense is where Roselia can take a hit and get more opportunities to Spike. Generally max out HP there. The SubSeeder could go Timid if you want more Speed, but there's really nothing in NU to outspeed in that range between Timid and Modest, so Modest is preferred.
Checks and Counters
Swalot doesn't care much about anything Roselia can dish out. Sunflora will switch in easily. Kecleon's Special Defense allows it to come in and hit hard with Returns and / or Trick you. Pidgeot can switch in on anything save Hidden Power Ice, but the Choice Band version dislikes being Leech Seeded. Delibird, while very uncommon, can Spin your Spikes and hit back with either Flying or Ice moves.
Note how Roselia may carry GrassWhistle, which can put your counter to sleep, meaning you'll need Sleep Talk or some decent switching around first.