I need to translate the phrase "hard to kill" from English to Latin. Does anyone know what that would be?

"ferreus neco" came up in a translator from English to Latin.
When "ferreus neco" is translated Latin to English it came up as "made of iron to slay"

Neco means: to kill, slay, put to death
Ferreus means: of iron; made of iron or like iron; hard, unfeeling, cruel; immovable, firm.
Or
Evigilo meaning: to wake up; to be awake, be vigilant, to watch through, pass in watching; to work hard at

Evigilo could be a better word because of its humanistic qualities. If you're planning on getting a tattoo it also would look good.

More latin words meaning Hard (http://www.archives.nd.edu/cgi-bin/lookdown.pl?hard)