radar, sonar, spider sense, electrocuting armor and or force field, running away over a trap door, then when the opponent falls through, fill the trap with cement. Robot, attack dog, weapons that affect 360 degrees (spin like a top while holding a mace).

Is it - you can't look at them in person? You can't look at the face? Is it the light from the opponent that affects you? Could you wear a 3D headset and have a filter blur the image? Or does the bad guy project something that blinds you?

The opponent must not be Paige (Blindspot) Madison, I guess - although I don't know how a mirror would deal with her power...

I'd say the protagonist might have to tie a blindfold over his eyes and rely on his other senses, as long as there is a risk of seeing this foe.

The protagonist would have to lure the foe to a place where a trap can be sprung. Perhaps explosives might be set to trigger from a tripwire or something, if the protagonist is willing to use potentially lethal means to end the threat. Or perhaps the trap can be in the form of a hidden pit, so that when the enemy walks over something over the ground, a built-in trap door can be pulled, causing the enemy to fall into the pit. If the enemy is running in pursuit of the protagonist, heedless of any potential traps, the enemy might fall into a pit loosely covered with branches and ground cover, but that kind of trap doesn't work as well if they're walking, since they might suspect a trap and walk around it on solid ground.

Of course, the protagonist could also use a double-bluff, using a shallow pit with loose ground cover to allow the enemy to detect it. Then, after they avoid it, they feel a sense of security, thinking the trap has been easily avoided, only to fall prey to the REAL trap immediately after -- perhaps the sturdy trapdoor that is much less easily detectable with a couple of inches of dirt or sand over it.

These ideas all assume that the protagonist will have enough time to fully prepare these kinds of traps, of course. There would have to be some time for the freshly dug-up dirt to dry so that the trap is less easily detectable.

The protagonist could also use long-range weapons, and if the resources are available he could have help from other people to set up his traps and such.

Along with any of the above ideas, the protagonist might also employ blinding light, loud sounds, and simple illusions to distract his foe's attention from the real threat he's set up.

Anyway, those are the routes I would explore.

OK, I like radar sense/spider sense or echolocation best!

These are all veru good suggestions, but let me add one more bit of pertinent information. The story is set in the year 1874.

If magic is involved, it shouldn't be too difficult. How about 'Ring of Fire' or 'Inferno' (fills the whole room with flame).

How about going to an asylum and recruiting an inmate to face the bad guy, and then ambushing the bad guy from behind with overwhelming force? The recruit is already judged insane by society - and who knows, facing the bad guy just might cure him...