Boss battles (particularly final boss battles) absolutely must have more than one way to beat them.
Say the goal throughout the entire game is to push this big red button that'll stop the end of the world or whatever. Between you and the big red button is the "thing-what-kicks-your-ass" final boss. You should be able to either dodge your way to the big red button, or kill the final boss first.
Of course, the off-the-beaten-path "get around the beastie" method should be made really really obvious but also give the impression that the other method(s) are viable solutions. For example, after making it obvious that you need to get to the big red button, make it so that some hits cause the beast to spew blood. (thus informing the player that the beast can be hurt)
Also, regardless of which method you take to get to the goal, there needs to be a MIB-esqe "last-stand" made by the beast. For example, if you get around the beast, *gasp* it chases you. Likewise, if you decide to kill the beast outright, it should lunge after you one last time before you win. (of course, you could draw this out into a full-blown "second form" of the boss fight) Bonus points if you make the player afraid to cross over the "corpse" of the beast after killing it the first time.