Well, in terms of "damaging a planet", while weapons like lazors or even ship-to-ship missiles may not be extremely ineffective at inflicting mass destruction on a planetary scale, a fleet that achieves decisive orbital superiority can easily afford to tow and drop a few large rocks onto the planet. Depending on the size and speed at which you fling these rocks, they can inflict very impressive levels of damage, and requires very low tech levels to do. So, any fleet, even a small fleet, if it can simply sit there unbothered for a few months, can conduct a devastating bombardment if it chooses.