How do I get my Reshiram to learn a Water/Grass (prefarably Grass) type Hidden Power?

Question

Also, how do I get it to have a base power as high as possible? It is Level 100 and has the following stats:
Nature: Quiet
HP: 369
Attack: 290
Defense: 225
Sp. Attack: 386
Sp. Defense: 252
Speed: 198

Comments

One idea is to put it on the GTS for another Reshiram and then repeat until you get your desired Hidden Power.

Answer 1

Pokemon Hidden Powers are determined by its IVs. So if its IVs aren't compatible with Hidden Power Water or Grass, you're out of luck, sorry.
I would say breed it to get one with the right IVs, but you can't breed Reshiram.

IVs also determine the type and strength of the move Hidden Power. In Pokemon Platinum, HeartGold and SoulSilver, you can find out what type Hidden Power would be for any Pokemon by showing it to a man in the game corner Prize Exchange building (Veilstone City for Platinum, Celadon City for HG/SS).-From the IVs section of this site

Answer 2

Hidden Power's type is determined by a Pokemon's IVs, which are predetermined before the Pokemon is caught/received and cannot be changed once caught. So basically, if your Reshiram has the IVs that cause Hidden Power's type to be Water or Grass, that's one step of the way done. If not, you can't change a Pokemon's IVs, so your Reshiram cannot learn that type of Hidden Power. For 70 base power, your Reshiram's IVs must be:

HP    Attack    Defence    Sp. Atk    Sp. Def    Speed
31    30        31         30         31         31    

*the above is for a Grass type Hidden Power.

HP    Attack    Defence    Sp. Atk    Sp. Def    Speed
31    31        31         30         31         30

*the above is for a Water type Hidden Power.

Source: Tested on PO/Experience.
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For further reading, see this. To check a Pokemon's Hidden Power type on side note, visit the Mistralton Pokemon Centre in B/W and speak to the man. In B/W2, it's at the PWT.

Answer 3

For Hidden POwer, you have to have certain Individual Values in each stat, and they're determined when the Pokemon is recieved/encountered. You can check a Pokemon's Hidden Power type by someone in, I think, Iciruss City.