The copter fight on the bridge platform? Not too tough, stick several high-powered arrows into it as it first approaches, and again once it emerges from underneath(you can really wallop it in the beginning if your aim is true). Then move back towards the ammo crate on the end of the bridge which can be used for cover against its assaults if you position yourself accordingly. Just pop out ot shoot a barrage of arrows when the opportunity strikes.
Oh and get accustomed to rolling away from missiles as they get close(you can usually plug an arrow or two into it immediately after a successful roll). Pretty much how I got through the fight unscathed on Hard.