First off, you should know how type synergy works. Say you have 1 Water (or Flying, or even both types) Pokemon out in battle and a Ground Pokemon waiting in the back. If you have a feeling the opponent will use an Electric move on you, you can switch out the Pokemon you currently have and bring the Ground Pokemon in to absorb the attack (Ground is immune to Electric in this case). This also works on other types, but it's all about you knowing the type chart.
If you want to know about good defensive cores, here's an example: Ferrothorn and Jellicent. One Pokemon resists the weaknesses of the other (Jellicent is immune to Fighting and resistant to Fire; Ferrothorn resists Water, Electric, Dark and Ghost).
Would you mind posting a team so I can help you further?