@Cyberspeed
You get a Turtwig, which can never use an attack which is super effective x2. After soloing any 2 gyms (not just the leader, no retreating to the Pokemon center in between), this restriction will go away. If you choose to evolve now, it will come back as no super effective moves at all.
If you solo two more gyms as a Grotle, the restriction will be reduced again to 2x super effective. If you choose to evolve again to Torterra, you may never use any super effective or 1x effective moves.
If you solo another three gyms, all restrictions disappear, and are replaced by Torterra must solo 10 pokemon from the Elite Four (not the Champion).
If you do not evolve fully, or do not solo the final three gyms, your Turtwig/Grotle/Torterra must solo one Pokemon from each Elite Four member.
If you manage to get a restriction-free Torterra, you are amazing, and have my eternal respect.
Alternatively, you may do this challenge with a Piplup or Chimchar, but Turtwig is preferred.