I would replace Solar Beam with either Seed Bomb or Wood Hammer as your main Grass type move.
Torterra has higher Attack (109) than Special Attack (75), not to mention that Solar Beam is a two turn move (unless you give Torterra a Power Herb or use it in sun), needing a turn to charge before it hits. In which case your opponent could just switch out into a Pokémon that resists Grass such as Heatran.
![Torterra](https://www.pkparaiso.com/imagenes/xy/sprites/animados/torterra.gif)
Hope I helped :)