Level 100 Statistics (see level 5, 50, 100)
At first glance, many people will doubt Roselia's effectiveness in UU due to its frail physical defense, low Speed, and the existence of better Grass-types. However, if you take a closer glance at its movepool, you will find that it has a interesting variety of moves that give it its own special niche. Unlike the common Grass-types, Roselia has access to the ever-so-useful Spikes, in addition to Toxic Spikes, and the ability to absorb status with ease thanks to Natural Cure. Roselia also comes equipped with a high base 100 Special Attack, which ties with the omnipresent Venusaur. Last but not least, Roselia's usable base 80 Special Defense gives it plenty of opportunities to set up deadly entry hazards on helpless foes. As long as you abuse its niches to their fullest, Roselia can be well worth the use.
If your team is in need of a Pokemon that is able to set up Spikes and take on many defensive threats, Spikes Roselia may indeed be the Pokemon for you. Roselia is one of two Grass-types with access to Spikes (the other being Cacturne) and has the typing and Special Defense necessary to set up Spikes on many Pokemon. Milotic, Registeel, Chansey, Rotom, and even some Venusaur (although it may require the -2 Special Attack drop from Leaf Storm first) can all be Spiked upon with ease. If Roselia's HP drops too much, Synthesis or Leech Seed can be used to restore health, while its ability, Natural Cure, cures any status ailments afflicted upon Roselia. Stun Spore can be a huge boost for Roselia by paralyzing offensive threats, especially since there are no other viable status moves available (Sleep Powder and Spikes are illegal together). Common switch-ins to this set include Arcanine, Moltres, and Mismagius, all of which are extremely hindered and almost unusable after being paralyzed with Stun Spore. To prevent Roselia from being stopped completely by Taunt, Energy Ball is used in the last moveslot. Energy Ball also deals impressive damage to many common Rapid Spinners, including Donphan, Kabutops, and Blastoise.
Synthesis is the preferred option over Leech Seed, as Synthesis gives Roselia a reliable recovery move that can't be blocked by immunities, missing, or Substitute. However, Leech Seed can force more switches, giving it more time to use Spikes. The decision ultimately comes down to how long you wish to keep Roselia alive. If its purpose on your team is to set up Spikes whenever possible and just sit as a filler, Leech Seed is probably the better option. However, if you want Roselia to be able to stay alive and be a decent answer to many Rapid Spinners as well as other special attackers, Synthesis is much better. The last attack should usually be Energy Ball to deal with Rapid Spinners, but Sludge Bomb is the stronger STAB attack. However, Sludge Bomb has many problems as an attacking move, allowing Pokemon such as Rhyperior to set up easily on it and giving Milotic an easier time surviving and eventually damaging Roselia.
Team Options & Additional Comments >>>
This set takes a much more offensive approach that is still designed to set up entry hazards. Egg move limitations prevent Roselia from running certain move combinations, meaning that which form of Spikes you want to run will influence the rest of the moveset.
Sleep Powder and Leaf Storm, both of which are illegal with Spikes and are meant to be used with Toxic Spikes, turn Roselia into a fierce weapon that can force switches, giving it even more chances to use Toxic Spikes. Leaf Storm has 210 Base Power after factoring in STAB, making Roselia a frightening threat. The last slot is saved for Sludge Bomb, which gives great neutral coverage all-around. Spikes is an alternative over Toxic Spikes for laying the hurt on every grounded Pokemon, while not being easily absorbed by Poison-types, unlike Toxic Spikes. If you opt for Spikes, then running Synthesis and Energy Ball over Sleep Powder and Leaf Storm, respectively, is a must. Synthesis gives Roselia a instant recovery move that allows for more set up time throughout a match, while Energy Ball enables Roselia to deal consistent damage overall.
Team Options & Additional Comments >>>
Every viable option for Roselia is mentioned in this analysis. The only moves with some use that are not given a mention are Cotton Spore and Aromatherapy. However, there is almost no need to temporarily lower an opponent's Speed with Cotton Spore when Stun Spore will permanently disable it, and Roselia cannot waste turns using Aromatherapy, especially when there are much more capable Pokemon that can use it.
Checks and Counters
Roselia's low defenses and average Speed give it a disadvantage against many Pokemon. Fire-types, Flying-types, and hard-hitting Pokemon in general can easily kill it. Pokemon that set up, such as Mismagius and Swords Dance Scyther, can also easily kill it and get free turns to set up. If a Rapid Spinner manages to spin, then Roselia becomes almost dead weight, as it will be unlikely for it to re-set up its entry hazards again, given the scenarios.