Since no one has properly answered your question, because they chose to instead of work with the one you have, basically told you to go find a new girafarig with a better nature, so instead of doing that, I am actually going to work with the Girafarig you have there...
Impish is Sp Att- Def+ which means you probably will want to go Physical...
Girafarig@Leftovers
Thunder Wave (Edit: This is an Emerald Move Tutor that you can get over and over if you collect enough BP)
Stomp/Return (Learned Move/TM)
Shadow Ball (TM)
Earthquake (TM)
If you want you can put in Iron Tail (I never have much luck with its accuracy but maybe you do...) and also Reflect and/or Light Screen are also options... but Thunder Wave + Stomp is cheesy enough to do you decently in the Battle Frontier... If you don't like the possibilities of Flinching opponent over and over then you may want to go with Return for more power instead
Throw your EV training into obviously Speed and Attack, but you can give some into HP if you so desire...