Question

where is he/she

Answer 1

In Black 2 and White 2, the Move Tutor that can teach ultimate moves appears outside the Pokémon World Tournament building, and will teach it to all five fully evolved starters.

Source: Experience clarified by this:
http://bulbapedia.bulbagarden.net/wiki/Frenzy_Plant_(move)#By_Move_Tutor

Comments

what do you mean all 5 starters

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all 15 starters

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@ Max Juniper: The fully evolved starter Pokemon, like Typhlosion and Samurott for example.
@ TBB: Only the fully evolved ones actually.

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ahh yes I forgot

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It's 15. Your source is related to Frenzy Plant, which only 5 starters can learn.

Answer 2

He is in the Pokemon world tournament to the right of the main building

Answer 3

In Black 2 and White 2, the Move Tutor that can teach Ultimate Moves appears outside the Pokémon World Tournament building.

Source