Level 100 Statistics (see level 5, 50, 100)
Even though she is not fully evolved, Roselia becomes a great asset to RU teams due to Eviolite. Eviolite makes her a great wall, boosting her mediocre Special Defense and pitiful Defense to formidable levels. Roselia also possesses a great support movepool that includes Spikes, Toxic Spikes, and Sleep Powder. Her pluses don't stop there; Roselia's ability, Natural Cure, is also a boon, as it allows her to shake off status effects and even cure herself from Rest upon switching out. Also, unlike other defensive Pokemon, Roselia boasts a decent base 100 Special Attack stat, which allows her to strike hard with a powerful STAB Giga Drain or Sludge Bomb. With an ideal support movepool, great defensive capabilities, and many other beneficial attributes, Roselia is certainly a Pokemon who will successfully fill the role of a defensive supporter.
With Eviolite and base 80 Special Defense, Roselia makes for a great special wall. Because of this, she is able to take minimal damage from some of the most frightening special attackers in the tier, including Accelgor, Lilligant, Manectric, Rotom, Rotom-C, Sceptile, and more. As Roselia will usually get a free switch into one of these Pokemon, she will be able to get up one layer of Spikes. Roselia's STAB moves are vital to this set, as they prevent certain threats from being able to harm her and her team. Giga Drain is extremely important, as it hits bulky Water-types and restores a bit of HP, which is useful as Roselia cannot utilize the recovery from Black Sludge. It is also important for checking Omastar, as Roselia can survive a +2 Modest Ice Beam from it and OHKO it with Giga Drain. Giga Drain also prevents common spinners, namely Kabutops and Sandslash, from spinning away Roselia's Spikes. Sludge Bomb is Roselia's secondary STAB move, hitting Grass-types such as Sceptile, Tangrowth, Lilligant, and Rotom-C for super effective damage. It also has the chance to poison the opponent, which could be useful for hitting common switch-ins that are immune to Toxic Spikes, such as Mandibuzz and Cryogonal. Synthesis is the main option for the last slot as it is reliable and doesn't force Roselia out after using it. Rest can be used instead to fully heal her and is more reliable against rain teams, plus she is able to rid herself of sleep upon switching out.
Team Options & Additional Comments >>>
Roselia has a variety of other moves she can use depending on her teammates. Hidden Power Fire is optional to hit Steel-type Pokemon that would otherwise wall Roselia. Leaf Storm can be used instead of Giga Drain to hit certain threats such as Spiritomb harder, but Roselia will become setup bait after the decline in Special Attack. Venoshock is an interesting option to use over Sludge Bomb, especially if Roselia is using Toxic Spikes, as it has a much higher Base Power against a poisoned opponent. An offensive set is an option to exploit her base 100 Special Attack; Sleep Powder, Leaf Storm, and Sludge Bomb with a Life Orb could make Roselia into quite a formidable threat.
Roselia has quite a few other options in non-attacking moves as well. A SubSeed set is viable, and she can use Sludge Bomb to keep Grass-types that are immune to Leech Seed at bay. Sleep Powder is another good option, as it puts the opponent to sleep, which could allow her to set up another layer of Toxic Spikes. However, keep in mind that Sleep Powder and Spikes are illegal together, so Roselia will be forced to use Toxic Spikes. Finally, Aromatherapy is another great support move as it clears Roselia and her team of status effects. She can also use it with Synthesis so that she does not have to use Rest and switch out to get rid of burns or paralysis.
Checks and Counters
Steel-types, Ferroseed and Klinklang in particular, take basically nothing from Roselia's attacks and can set up on her. Fire-type Pokemon, such as Moltres and Entei, can OHKO Roselia, while she can't do much back and is forced to switch out. Flying-types such as Sigilyph and Mandibuzz are also a nuisance for Roselia, as they resist her STAB Giga Drain and can annihilate her with a boosted Air Slash or a powerful Brave Bird. Specially defensive Mandibuzz takes practically nothing from her attacks and can even Taunt Roselia, preventing her from healing or setting up entry hazards. Cryogonal is one of the most solid counters to Roselia, taking almost nothing from her attacks due to its high Special Defense and either spinning away Spikes or hitting hard with a STAB Ice Beam. Even resisted physical attacks will obliterate Roselia; for example, +2 Feraligatr's Waterfall will 2HKO her, and that same Feraligatr's Return will OHKO. Most Psychic-types have an easy time against Roselia as well because they can simply blast away at Roselia's weaker defense stat with Psyshock. Finally, Roselia hates getting hit by Trick because she becomes much less bulky after she loses Eviolite. Thus, be careful when switching her into Rotom and Rotom-C.