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

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.