i can't kill the black element at bevelle. he keeps on casting flare and ultima which does 9999 damage no matter what. could someone help me? also could you give me a strategy on the other bosses?

What Dress Spheres and Garment Grids are you using? What stratergy do you use to try and take the boss down i.e. two tanks one healer?

Either equip star bracers(auto-reflect)/use star curtain/reflect right at the start of the battle - this stops all attacks except ultima, and absorb(used only when it is low), then kill it as fast as possible - use magic attacks, def-ignoring attacks, maybe gunner's tableturner(damage increases on enemies with high def),
or you can use Yuna's special dress - have right pistol use mirror petals as soon as possible, have Yuna use barrier(nullifies 2 magic attacks - this will stop ultima on her,though it won't save the pistols), then on right pistol's other turns, lower black element's magic def - it will kill itself with reflected flare's

thanks but i might try a different strategy because i watched some video where this guy used white hole and another att

Charon hits for 2x the user's max hp. It will kill black element if you are a high enough lvl for dark knight to have at least 5k max hp;black element's hp is 9999.