Torterra

Torterra is an acceptable choice as a Grass attacker; it used to be solid, but it has now trailed behind Shadow Grass attackers and the extremely deadly Kartana. Its Grass/Ground typing makes it a mixed bag of a Pokemon. Its best moveset is Grass Type, and only three types are weak to Grass: Water, Ground, and Rock. 

• When facing Water Types, Its Ground Typing neutralizes Grass' natural Water Type resistance. It also imposes a 2x weakness to Ice, which is a type that many Water Type raid attackers tend to pack
• When facing Ground Types, it generally has a fairly by-the-books matchup. However, it does have the advantage of resisting Ground Types, which is something that Venusaur and its variants wish that they could say.
• When facing Rock Types, it actually has a great advantage. While Grass doesn't resist Rock, Ground does, meaning Shadow Torterra instantly has a leg up on the competition

Overall, Shadow Torterra is a strong Pokemon, and raising a high-IV individual is a solid investment.

Best PvE Offensive Moveset

• Razor Leaf + Frenzy Plant makes Shadow Torterra a formidable Grass-type.

• Solar Beam can be used if you don't have access to the Community Day exclusive move, though it's much worse.

• Earthquake has STAB; it’s usable with Razor Leaf, or Bite if Grass is resisted.

• Without STAB, there aren’t any edge cases where Stone Edge should be used.

• Sand Tomb has very poor damage output for PvE. 

Best PvE Defensive Moveset

• Razor Leaf has much more power than Bite, and STAB to boot.

• With its other moves all being 1-bar charged moves, Frenzy Plant can at least damage some Pokemon before it dies.

• Of the remaining moves, a surprise Stone Edge is nice coverage.

• Solar Beam and Earthquake, though stronger, are far too easy to dodge.

• Sand Tomb can be used frequently as a 3-bar move, though it's not hard to dodge. 

Razor Leaf takes advantage of STAB to deal massive damage, at the cost of the offering the worst energy generation in the game. Bite is generally an inferior move due to a lack of STAB and also worse damage output stat-wise, but an off-typing can catch opponents by surprise in certain formats. 

Frenzy Plant is very powerful and hits even harder with STAB, but is somewhat redundant due to the high damage output of Razor Leaf. Sand Tomb is recommended to for its low energy cost and debuff utility. Stone Edge is good for coverage against all of Torterra's weaknesses, though poor energy gains makes it difficult to use. Earthquake benefits from STAB and provides wide coverage against numerous typings, but 65 energy is a lot to ask for from Torterra. The revamp of Return during the Dec 11, 2019 rebalance makes it cost too much energy to be reliably accessible.