This man runs Rhoemer's fake monastery, which contains both illegal experimentation facilities and sprawling catacombs used to imprison their unwilling test subjects. Make no mistake; Cavanaugh is a monster.
After he barks a few orders at a scientist in a cutscene, you'll be starting the catacombs mission. You're lucky -- this is one boss that is arrogant enough to keep his back turned to you, making this more an assassination than anything.
As a result, you really are simply choosing how you want to do away with him. Sadly, the air taser seems to be out of the game -- for some reason, it alerts the scientist when Cavanaugh starts screaming.
A sniper rifle shot to the head is recommended, for you'll be far, far away and perfectly safe.
More honorably, you could sneak up on him (make sure you stick to the wall he faces away from) and execute him with a pistol.
If you want to make him suffer, a leftover gas grenade may be pretty saucy. If you only have one, however, it's recommended you save it for the two guards further in the level.
Things to remember
This is a stealth mission. Don't kill Cavanaugh in a way that would alert the guards or the scientist. Don't kill him too quickly, either; the scientist will see. Give him a grace period of five seconds if you're going to shoot Cavanaugh, ten if you're going to gas him.
Cavanaugh is arrogant enough to be a poor guard, but not enough to walk around without a flak jacket. If you choose to do away with him via bullets, utilisez un headshot s'il vous plait.
If you wait too long, there's a good chance Cavanaugh will glance over and see you. You don't want a gunfight, not with a man when you're in a tight corridor and he has a PK-102! Step it up.
↑"This is Rhoemer's Stronghold, a place where he keeps prisoners. Undesirables. Alec Kabanov runs the site for Rhoemer. He kept Phagan supplied with human test subjects for various forms of the virus. Now Phagan is being held prisoner himself, probably somewhere in the catacombs" - Thomas Markinson