Hfrhyu

The story was published in the mid 1960s, around 1967-68 in Analog or The Magazine of Fantasy & Science Fiction. I delivered both subscriptions to a young guy in Belfast when I was a paper boy and he would pass me his read copies when I delivered the new ones.

The character is bio-engineered, a bit like the Terminator, but with normal sentient self awareness, working as a kind of vigilante detective who can, because of extremely advanced biologically engineered senses, see the otherwise cloaked 'normal' aliens and kills them by whatever means possible.

Like the story Eight O’Clock in the Morning by Ray Nelson, first published in the November 1963 issue of The Magazine of Fantasy & Science Fiction. Which was later made into a film in 1988, They Live, by John Carpenter. He also has narrow escapes, when he crushes one against a wall with a car when he's ambushed.

He also escapes from them when he's trapped on a ship and has to swim a mile under water (with a stainless steel endoskeleton - go figure) to escape them. I thought it was The Stainless Steel Rat by Harry Harrison but apparently not...

This is A Plague of Demons' by Keith Laumer

From fantasy literature review.....

http://www.fantasyliterature.com/reviews/a-plague-of-demons/

A Plague of Demons is narrated by John Bravais, an agent who is asked to come to Algeria to meet his old friend Felix Severance, of the CBI (a combination of the CIA and FBI, we can only infer).

Bravais learns that for many years, war combatants around the world have been going missing, seemingly vanishing from their fields of battle. Severance asks him to investigate, and it is not long before Bravais witnesses, during a campaign between Moroccan and Algerian troops, just what has been going on: Seven-foot-tall, skull-faced, occasionally bipedal aliens, vaguely canine in appearance, have been surreptitiously killing soldiers on Terran battlefields, slicing out their brains and storing them in vitro for some reason unknown!

For the next 2/3 of Laumer’s book, the aliens, and their human cat’s-paws, chase Bravais from Algeria to Jacksonville, Florida, and then on to Coffeyville, Kansas and to Chicago; a nightmarish pursuit that Bravais survives only because of the PAPA (Power Assisted Personal Armament) modifications that Severance had made on his body, turning him into a superwarrior of sorts.

But matters grow even more nightmarish for Bravais in the book’s final 1/3, in which he is captured by the aliens, has his own gray matter removed, and awakens on an alien moon in the midst of a battle, his brain being used to power and control a 70-foot-high, massive supertank, in concert with other tanks being propelled by the minds of Earth soldiers from many nations, some from as far back as 1,000 years ago