Personally i hate the idea of any sort of uber boss passive or something along those lines, a powerful channeled invulnerable
spell may be interesting, i may think of something later, but what i personally loved was in God of War Ares battle
(any one play) is that in a cut scene he sucked you into an alternate universe (a different arena) where you had to
complete a puzzle, a relatively simple protect mission but still it was pretty fun, unique and changed up the gameplay.
Something like that as the boss' approache 5% hp in which a cut scene appears and the character/s get blasted,shot,
sucked, forced etc... into a portal and a new area where a challenge awaits they need to complete before returning. As
of yet i have no ideas ( i will think on this later) but after fnishing the said puzzle (could be any objective) you
could successfully return in a cut scene and the boss' could be "oh wtf" and then the charcters could instapwn the rest
of their 5% hp in one cool hit spell etc...
As their are two bosses, once one reaches the 5% hp he goes intto a invulnerable state and naturally you have to beat the other down to until he is on par health, then this procedure would be called.