Okay this is going to have to be a record for the longest time for a correct response with so many reads (over 1860 reads) on this site. Here it is:
To get 100%, you must change to The great Spirit or switch to the Gate Keeper Oogani before the Big Booli gives you the final death blow.
What I did was this: After talking to the Healer, that says my sacrifice will save many Oogani, I moved a little to the left of him & then faced right. I started spamming
to shoot the fireball, barely over the edge.
A little bit later, the screen focused down & showed the Big Booli spawning. I then ran & jumped over the edge. As soon as I jumped, I pressed
to go into spirit mode & to the Gate Keeper (I never actually switched to him.).
A second later, I got the Level-ending cut scene (I never even saw the Chief getting Attacked or Killed.) & there you go 100%