yoshinite, revive the first Fossil you successfully dig up in the Underground and nickname it either Zuni (Cranidos) or Cephy (Shieldon). Each of these Pokemon was raised by Pokemon of the opposite species (Zuni was raised by Shieldon, and vice versa), and as such they act much more differently than normal.
For example, Zuni became very passive, and as such, it can only know 1 attacking move ASAP, and only use it after being attacked. It must also stay in battle for at least 1 turn when sent in (no, the turn where you switch it in does not count). Don't worry, you can switch Zuni back out after that 1 turn is over. Also, despite being raised by Shieldon, whose pride is defense, Zuni always has trouble properly defending itself, and because of that it can't use Protect.
On the other hand, Cephy became aggressive to the point where its entire moveset must consist of attacking moves, one of which must be Iron Head (if it gets high enough in level). However, all Cranidos know when to run away, and Cephy adopted a simple system to accomodate this: If the enemy is heavier than Cephy (or is a Water, Grass, Ground, Fighting, or Steel-type), it must switch out, but must stay in until either it or its enemy faints in all other cases. (For reference, Shieldon weighs 125.7 lbs and Bastiodon weighs 329.6 lbs.) Also, all Cranidos (except, of course, for Zuni) prove their strength by defeating a powerful enemy, but Cephy will have to make due with soloing a Gym Leader/Rival/Galactic Commander instead. (Notes: You can use items on Cephy during the solo. Also, the above restriction can be ignored for this solo if necessary.
Have fun!