ice and rock
Ice type Pokemon in Pokemon Pearl are weak to fighting type moves, rock type moves steel type moves and fire type moves. This has not changed since the first generation (Pokemon Red and Blue/Green) when the steel type did not exist.
Steel type Pokemon are weak against Fire, Fighting, and Ground type attacks
Steel Pokemon is often weak against fire, water and ice attacks. But some attacks is too weak to inflict much damage, like ember or watergun
psychic type pokemon are weak against psychic steel and dark
Steel type Pokemon are weak against fire, fighting, and ground types. So if you want to defeat a steel type Pokemon or catch one, make sure you use super-effective moves that are weak against steel type Pokemon.
Fairy type Pokemon are weak to Poison and Steel type Pokemon.
Steel type Pokemon are weak to Fire, Ground, and Fighting, so those types of Pokemon will do well.
Porgon can be found in the trophy garden in Pokemon Pearl. Porgon is a normal type pokemon weak to fighting type pokemon but immune to ghost type.
Ice and Rock pokémon are weak against steel". Steel is weak against Fire, Fighting and Ground
steel is weak to fire and water
Rock-type Pokemon are weak against Water, Ground, Steel, Fighting and Grass-type attacks.
Bastidon is both Rock and Steel type. Both Rock and Steel types are weak against Ground and Fighting. Either will have a x4 type advantage over Bastidon.