you have to select it when you start on pokemon diamond, from experience
Somehow...
u need to train your Pokemon to the high 50s and low 60s or use your best statergy to do the by a Pokemon strategy book
Pideot Lv. 70 Giratina Lv. 70 Infrenape Lv. 70 Heatran Lv. 70 Dialga Lv. 70 Haunter Lv. 70 That's my team and I beat the whole game with them. legendaries are cheap, also mismagius instead of haunter impresive but my team r beter this is my finishing team infernape lvl 100 moves: fire blast, close combat, overheat and blast burn palkia lvl 100 moves: aura blast, draco meteor, dragon claw and special rend arceus lvl 100 moves: x scissor, fly, surf and judgment togikiss lvl 74 moves: metrenome, fire blast, flame thrower and fly staraptor lv 63 moves: brave bird, focus punch, close combat and fly lucario lvl 83 moves: aura blast, waterfall, close combat and dragon pulse well that my best team in pokemon diamond ( i have other lvl 100s but i dont use them much like i have regigigas lv 100 and charzard lvl 100)
First boss Aaron, Bug type Pokemon Yanmega Vespiquen Drapion Scizor Heracross Second boss Bertha, Ground and Rock type Pokemon Sudowoodo Rhypherior Hippowdon Gliscor Golem Third boss Flint, Fire type Pokemon Rapidash Infrenape Drifblim Magmortar Houndoom Fouth boss Lucian, Psychic type Pokemon Alakazam Bronzong Espeon Mr. Mime Gallade Champion Cynthia Garchomp Spiritomb Lucario Milotic Roserade Togekiss Levels are in the 50's and 60's but after you defeat stark mountain and the northern area above the Pokemon league, the elite four's Pokemon level up and they are in the 60's & 70's so be warned because I came unprepared and I barely beat them. Hope I helped =D