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.
Shed Shell can be a somewhat useful item, as Roselia is dreadfully weak to Dugtrio. If your team relies on Spikes, Shed Shell may be a good item to use, if only to ensure Roselia's safety. 60 Speed EVs allow it to outspeed Hitmontop, which sometimes might allow Roselia to finish Hitmontop off with Energy Ball before it can Rapid Spin your Spikes away. You could add to bulk and make Roselia's EV spread 252 HP / 252 SpD / 4 Spe, but there is no reason to not try and take advantage of outspeeding a Rapid Spinner. Roselia could run 96 Speed EVs to outspeed Jolly 252 Speed Rhyperior, although the current spread will outspeed Adamant 252 Speed Rhyperior. The choice depends entirely on how much bulk you are willing to sacrifice. Rest can be used over either of the healing moves, giving Roselia full recovery at very low cost due to Natural Cure's status healing ability. However, it forces Roselia to switch out and back in to be useful again, giving the opponent free turns as well.
Offensive teams truly benefit from Roselia's Spikes, as they turn many 2HKOs into OHKOs. It can also help cover many holes found in offensive teams, such as weaknesses to Registeel, Chansey, and even Rotom. However, Roselia is not much of an attacker and should not be used to keep the momentum rolling; instead, use it set up more layers of Spikes to ensure that other sweepers can clean up. Stall teams appreciate Roselia very much, as it provides a valuable check to many Pokemon that threaten stall-based Pokemon. A partner such as Milotic to take Fire-type attacks without much harm can help Roselia obtain extra turns for Spiking.
Roselia's major issue is its general lack of stats. Being a pre-evolution, it unfortunately doesn't have the Speed and defenses to defend against many powerful Pokemon found in UU. Every Fire-type and any strong physical attacker can easily OHKO Roselia. It must also beware of Hitmontop that carry Foresight - if Roselia switches to a Ghost-type to try and block Rapid Spin, Hitmontop can use Foresight to KO them, while if Roselia stays in, it risks being taken out by Hitmontop due to its low bulk. Therefore, bulky Water-types and a double Ghost strategy can be useful. Milotic can safeguard Roselia from Fire-types with ease due to its huge special bulk, while utilizing two of either Rotom, Spiritomb, or Mismagius can help block Hitmontop's spin easily.