Usually, the most attractive planets are guarded by space monsters.
There are six species of space monsters. Four of them have the serious problem of having just one weapon ready to deliver significant damage. This is what makes them rather unfortunate creatures that get wiped out quickly when standing in the way of the players expansion plans than a serious obstacle.
The key to eliminating foes with just one strong weapon is attacking with a bunch of small ships. Since the monster is just able to kill one ship per combat turn, the remaining ones can easily deliver the damage needed to destroy it without the need for expensive protective gear.
The most cost-effective approach is to take frigates with battle pods and load it with MIRVed missiles in racks of two rather than five. Eight of these will kill almost every minor monster and are cheaper to build than the colony ship needed for subsequent colonization. There's one exception, though. The Hydra needs at least 12 frigates or one "heavy armor" cruiser to soak up some damage because of the three independently targetable plasma weapons.