Professor Oak is a world famous pokémon scientist who lives in Pallet-Town. He gave Pikachu to Ash in order to let him begin his adventure. In parallel with his studies, he takes care of all the pokémons the trainers send to him (so to say, really much).

Difficulty : *****

Tested, logical.
If you are stuck, you can use edge-logic, especially with the first 3 and the 2 from row R5.
Good blackenage.