Douglas Harper's Etymology Dictionary
Wiktionary
a. Not killable; unable to be killed.
Usage examples of "unkillable".
For the Amar, floating ghosts were the most horrible of monsters, creatures unkillable that sucked the souls from the bodies of helpless, hapless warriors foolish enough to venture within the mists.
It was unkillable: a bullet just passed through it, and an explosion would just scatter it and create a bunch of small doughballs, which would each then immediately roll off in different directions, consuming and growing.
Rahff and Chak have been telling her you're unkillable, but I don't think she believes them.
Two of them old unkillables and Talley the only repple with too little experience-and a shame Fielding has la grippe or whatever it is.
You know, we minionettes are almost unkillable by normal means, but that thing—I’d like to meet it again.