So the user of the Armored Titan can transfer his consciouness to his body before being attacked, then transform into a Titan, and then heal his body. How can he be killed then?
Manga spoilers not possibly covered within the 3rd Season below.
There are three ways.
1.) Make him lose his will to live
In Chapters 102 - 103,
when Eren infiltrated Marley, he had a talk with Reiner. During this, Eren transformed and Reiner tried protecting Falco but was also injured. His injuries, however, did not heal. Falco remarked that, to quote, 'if he has the power of a titan, any injuries should automatically heal themselves..So long as he has a strong will to live...'
During their talk,
Reiner was showing remorse for everything he's done against Eren. Reading those chapters, you could say that Reiner lost his will to live after Eren, resulting to his injuries not healing.
Therefore, one way to kill the Armored Titan is to make him
lost his will to live so that any injuries afflicted cannot heal.
2.) Wait for Ymir's curse to take into effect
Because of Ymir's curse, Titan users' lifespans only last for 13 years so another possible way is to wait until the Titan lived it's 13th year, then he'll die. The power will then be passed on to a random subject of Ymir born after his death.
3.) Make a Titan eat him
Using a Titan injection, a person can be turned into a Titan. Then while the Armored Titan is still in human form, if they can somehow subdue him in human form, the Titan can eat him and the power will be transferred.
This was how the power was taken forcibly from
Frieda, Marcel, and Bertolt.