Not sure about the capabilities of the Ing, never got very far into Metroid Prime 2. IIRC however, they can't exist in the "normal" world without possessing a host. Kill the host creatures, Ing are helpless.
Here's where my knowledge really dips into Mwars fanon.
So the ing use these things called
dark seeds. They implant the seed in the planet, and after a few months or so the planet is taken completely out of the host dimension and into the dark dimension. This gives a huuuuge advantage to Ing defensive positions, as to even attack them one needs to transfer to a hostile dimension.
The one thing you brought up was host creatures being a weak point, so consider this.
On an imperium Hive Planet, psykers detect a large absence in the void moving closer and closer to the planet. After much investigation, local authorities would maybe say that it is a natural phenomenon. However, after a few months with a large void-phenomenon in orbit, entire legions of PD forces are converted by Ing insurgents. the very wildlife itself begins to turn on the planetary defensive positions. Void shields start failing, planetary guns go out of commission, or start firing on the Planetary Defense Fleet. The Imperial guard is thrown out of whack, and all their efforts go to waste on an enemy which does not even reside in their own dimension.
Then, the seed comes into the host dimension. A large organic leviathan, the Planetary Fleet is helpless to stop it from landing.
When it lands, it begins the conversion process. Over the weeks, slowly, mile by mile, the seed takes the planet into the dark dimension. Fleet actions are useless, as atmospheric phenomenon cuts off any form of help for the planetside forces. Eventually, the Imperium at large resorts to exterminatus. But when the phenomenon clears, the planet is no longer there.
Gone in only a few months, an entire planet snagged from the host dimension.
Rinse repeat for every solar system until world domination.