How do I beat the first boss (Barrett)?
Toss a yellow canister at him. Then while he's coughing and cursing you, throw one of the explosive red barrels at him.
If you're on higher difficulty levels than story mode, repeat as needed.
Very simple -- 7 non-stop zaps to the head (or 15 zaps to the body) with the Stun Gun will bring Barrett right down.
If you rush in and zap quickly enough, he won't even get a chance to shoot at you.