there's always the 'Silver Bullet' for every soldire, no matter how good he/she is. Even the greenest, n00biest civie has a chance of taking down an altimate soldier: usually be mistake or total blind luck, but it's always possible.
In Bossk's case, he most likely would get Fett somehow unaware, maybe snipe him or something. Fett's about the hardest man in the galaxy to catch anaware, and he's helped by such amazing tech it's that much harder, but if it was a situation where Fett wasn't expecting to run into Bossk *not on a hunt or something where he'd expect to run into compitition*, and Bossk was just after him for Revenge, then it's possible.
A complicated trap with explosives or something wouldn't work I don't think; Boba's helmet sensors and the like are too good at picking that stuff up. Even sniping would be hard in the SW galaxy, as must scopes probably have advanced targeting toys like lasers and/or other targeting mechanizems to lock onto a target and take the work out of judging distance and such; Fett's helmet would probably detect any of this as well. But low tech, something he's not expecting: either a grenade he can't get away from or a low tech sniper rifle/scope might do it....IF you're lucky.......VERY lucky
"You set a code to live by. I won't be wronged, I won't be insulted...I won't be laid a hand on. I don't do these things to other men, and I require the same from them."