unconquerable
assiduous
beyond control
brave
breachy
constant
continuing
contumacious
courageous
dauntless
defiant
determined
diligent
dogged
enduring
faithful
fearless
fractious
immutable
impassable
impenetrable
impregnable
inalterable
incontestable
incorrigible
indefatigable
indocile
industrious
inexpugnable
insistent
insuperable
insuppressible
insurmountable
intractable
intrepid
invincible
inviolable
invulnerable
irrepressible
irresistible
lasting
loyal
mettlesome
never-say-die
never-tiring
obstinate
impetuous
implacable
resolute
self-reliant
steadfast
stubborn
tireless
unabated
unflinching
unquenchable
untiring
unyielding
warlike