Stats:
Arboliva has an amazing Special Attack stat, having one of the highest attacking stat of any non-paradox Pokemon in the game. Its bulk is also good compared to other Grass-types, such as Brambleghast and Scovillain. Its Speed, as you mentioned, is low. However, its bulk is enough to remedy that. It also makes it good for Tera raids because Speed isn't really used since you can attack whenever you want. It also combines well with its ability, but more on that later.
Typing:
Arboliva's typing isn't exactly desirable, having a whopping 6 weaknesses and only 4 resistances. In addition to this, its offensive capabilities are hindered by its type as well, only hitting 3 types for super effective damage and having Steel resist both of its STABs. This can be helped with a few coverage moves, but we'll talk about that later. Overall, Arboliva's typing is outclassed by Scovillain's and possibly Toedscruel's due to its offensive capabilities.
Ability:
Arboliva has the ability Seed Sower. Combined with its low Speed, it can have boosted Grass type attacks because the opponent will most likely be hitting you first, setting up Grassy Terrain before you attack. Toedscruel's ability is awful, and while Scovillain's are alright, Seed Sower is better due to the healing combined with Arboliva's good bulk.
EDIT: As Fizz mentioned below, Seed Sower can be annoying due to the animations being slow, so you may not want to have it. Harvest can be used, however it may be tedious to grind for a hidden ability one.
Moves:
Arboliva's moves are fine. It gets most of its good moves through TMs, but this is the same for the other Grass types. It has Energy Ball/Giga Drain and Hyper Voice for STABs, and Earth Power, Dazzling Gleam, Pollen Puff and potentially Tera Blast for coverage, but that's about it. It can make use of these, though, with Earth Power hitting the Steels that it struggles to deal with, Dazzling Gleam defeating the Fighting types that it is weak to as well as hitting Dragons harder, and Pollen Puff dealing with other Grass types. Scovillain has a slightly lower amount of coverage, but its offensive typing is better, while Toedscruel has better coverage than Arboliva as well as a better offensive typing. As for support, Arboliva gets some nice moves such as the screens and access to Grassy Terrain(Which it doesn't really need but still) to help heal the team slowly.
So far, it seems that Arboliva is a good choice compared to other Pokemon, however I have only compared it to Grass types native to Paldea. Lets look at some other Grass types:
Tsareena:
-Better Moves
-Better Ability (Debatable)
-Evolves earlier (Learns Stomp at level 28 as a Steenee)
-Better Typing
Tsareena also has a similarly good attack stat and better speed. Its bulk is also respectable.
Breloom:
-Higher attacking stat
-Higher Speed
-Better offensive typing
-Evolves earlier
-Better abilities
-Access to Swords Dance
Its bulk isn't the best, however, and it does have a 4x weakness to flying.
Both of these Pokemon are faster Pokemon and have better moves.
In conclusion, Arboliva is good, however it has better opposition from non-Paldean Pokemon such as Tsareena and Breloom. Use Arboliva if you only want to use Paldean Pokemon, and use Tsareena/Breloom if you don't.
Movesets:
EVs are optional.
Arboliva @ Leftovers
Ability: Seed Sower
Tera Type: Grass
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
- Dazzling Gleam/Reflect/Light Screen
- Earth Power
- Energy Ball/Giga Drain
- Hyper Voice
Mega Drain ---> Energy Ball/Giga Drain
It gets its coverage quite late, so you can use Growth to power up your moves early game.
Miracle Seed can be used if you don't want to make battles even longer than they already are. It can also be used when you don't have Leftovers yet.
Breloom @ Toxic Orb/Punching Glove
Ability: Poison Heal/Effect Spore
Tera Type: Grass
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Swords Dance/Spore
- Drain Punch/Mach Punch
- Seed Bomb
- Thunder Punch
Bullet Seed ---> Seed Bomb
Spore can be used in place of Swords Dance until you get it, or can even be used as a replacement for Swords Dance altogether. The same applies for Mach Punch.
Toxic Orb is only available Post-Credits, so you might not be able to use Poison Heal as effectively. Loaded Dice can be used for Bullet Seed.
Tsareena (F) @ Miracle Seed
Ability: Queenly Majesty
Tera Type: Grass
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- High Jump Kick
- Play Rough
- Power Whip/Seed Bomb
- U-turn
Razor Leaf ---> Power Whip/Seed Bomb
Low Sweep ---> High Jump Kick
Play Rough is obtained quite late, so you can use a filler move until then.
Hope this helped.
