 how would McClain know about the weakness, though? It's not like McClain investigates the weaknesses of his opponents(ala Batman). And I doubt Dunn would go running around going "I AM WEAK TO WATER AHHH WEAK TO WAATAAHHH" Well after a double tap to the head does nothing but knock him out its fairly obvious something is wrong. From there McClain could run the gamut of damage types trying to find something that would work. Sooner or later McClain would have to arrive at the fact suffocation is the only way Dunn is going to die. It really doesn't matter how it happens as long as its suffocation. And really after McClain puts him you with a gun shot to the head its not to hard to keep him that way. Though that's assuming McClain doesn't do something stupid like tie him up and try to interrogate Dunn. 