The s-man sports a pair of glands on each of his wrists, each gland capable of producing a high-pressure stream of one chemical. Each chemical alone is mostly harmless, but when together, they instantly combine into a single highly caustic liquid, capable of destroying any organic material within seconds. It also clings to whichever surface it contacts. S-men can control the intensity and spray cone width for each wrist separately. Maximum range about five meters. Minimum range depends on just how ... brave ... the s-man is.
It's great against infantry. Causes immense pain to any exposed body parts of your enemy, putting them out of combat instantly. If they wore leather armor - now they don't. If they wear chain mail, you can still shoot through it, though you do need to dissolve some more layers of armor before you begin damaging the opponent's body. And even if they wore plate armor (rare), there are still exposed parts you can target - and any damage to the eyes inflicts a -100% modifier to accuracy. Recommended accessory: A shield or buckler, possibly of metal; a pair of modified gauntlets, with wrist-slits for the caustic spray.
One s-man vs. one horse rider - A horse is a formidable beast, if only for just a while. Caustic spray can make even the best trained warhorse writhe in pain and unable to walk, but they still need to withstand the first charge. A shield should be of great assistance to that end, preferably combined with clever use of the environment. With a little time to prepare it's also possible to just spray a thick mist of the substance to reduce visibility as well as serve as a psychological deterrent. One downside is that it presents an impassable terrain for the s-men as well.
Archers are still as much of an issue as they are for humans. A large shield is recommended. Enough caustic mist might turn arrows into tumbling arrow tips, but then again it might not act fast enough to do that in time.
In modern times - You might be able to kill a law enforcement officer unprepared to deal with you. Moments later, you'll be lying on the ground with a crippling gunshot wound and slowly bleeding out. Or, you might get lucky - the gunshot went into your wrists (or they decided to spare you of gunshots altogether), and now an officer is escorting you to a prison equipped to deal with s-men. His uniform sports a glass fiber mesh to deal with the caustic spray. He uses laminated glass for the helmet. You are forced to wear mittens resistant to your caustic ooze. They are woven in a way such that they can be firmly locked in place by police handcuffs...
And yes, you are being detained.