Glad you pointed this out.
The actual achievement in English is to destroy all Zerg structures before freeing all captive Protoss. There are a total of 3 sets of Protoss captives too free. So as long as you destroy all the enemy’s structures before you free the 3rd set, you should be good.
So I’d recommend that you play normally until you reach the 2nd Protoss captives and rescue them. This way, you will get a 2nd base and also some time to build up your forces. Once you deem it to be sufficient (and have saved), you wait for Kerrigan to move out before sneaking behind her to hit the most forward Zerg base.
Once you get this first forward base down, the Zerg basically comes to a halt. They will stop pushing the Terran base and you will get all the time in the world to mass up and then hit the other bases. The key to note here is not to rush too much because the Terran’s final base is rather well defended and should be able to take 3-4 waves at least.