Having trouble with boss fight in FF

Can anyone give me some tips on how to take down Barthandelus in ch 11, the book says to use delta to stagger then mystic tower and when he cast doom on you to go with relentless but in never works and the timer runs out and I die.

I'll try it, thanks.

post back if you need more help, i beat the game already.Somaticus

Nice it worked, ty

IMO he is more difficult than the final boss