How to make a legendary boss battle.
A - An outstanding boss
The shape of the boss is meaningfull. Some DragonQuest game missed this chance, some FinalFantasy do it right, Phantasy Star excel.
B - Multistage boss
Having the boss change shape/routine after a number of lost HP give you nice feedbacks.

(That was the previous form of the boss above)
C - Superepic music and background
You should save some energy to deliver the best in the end.
Now, a good idea is to have
an eroic music instead of that usual darkness/metal/goth music that's more celebrating boss strenght than your - it's time for the heroes to shine instead! They will use their best epic moves! A good example is OKAMI
D - Plot twist, reset of resources and restart
When everything seems lost, cutscene, something happen and the battle start again with refreshed resources (or even more of them).
Saint Seiya battles are exciting 'cause of that: everything is actually lost, then a new tech/unexpected help come from nowhere and the battle restart with more fair odds. Hype
E - Vegeta fanboys should be satisfied
While the hero is the focus of the overall plot, probably some player got his own best character - each one should have some weight in the final battle.
The final battle should naturally be the plot solution of every character. If the hero is fated to defeat evil, another could have a personal grudge, for example.
F - Avoid too much dialogue "inside" the battle
I'm hyped, excited and immersed in the action, to the point I'M the one delivering blows. Break the pace with too much dialogue lines after and before single actions and everything break apart - you can have a nice introduction and an inspired aftermath to chill down, but please let me play this moment my way (that's why I think MOTHER 3 final battle is atrociously annoying).
G - Avoid Z-Sword
The magical power appearing that very last battle, is the only power that can defeat the evil and you have a very easy time by using it - why I'm playing that final battle?!? Why I have played the entire game?! Some Last Bible game suffer of this (best summons gifted near the end totally free T_T After all that effort to come with my own demons...).
H - Final boss moment
Give the boss some moment to tell last lines, die epically and fade into nothing. I do believe too there's need for the whole "location" to shatter, crumble to dust, explode, change, whatever, to hint that defeat was indeed epic to a large scale.