Basic Attacks against Enemy Heroes deal an additional 1.5% max health in damage."
The luxury talent. If you can afford to not take Sixth Sense , you take this. Even against a team with smaller health pools, GK is a nice DPS boost. However, survivability is your #1 concern. As you've surely heard said many times, you don't do any DPS when you're dead.
NOTE: NEVER take Giant Killer if you took Thrill of Battle . Half of the reason to take ToB is the Evasion/Shadow Shield/Sixth Sense synergy. If you took First Aid, you may then consider Giant Killer if you're ahead..