Hey thanks for the replies so far

Some really interesting points made.
Just to add slightly to what I am describing the enemy is a human (if that makes a difference to this) and hasn't harmed Tidus etc. Writing this bit I am on it just "feels" wrong having Yuna act unmercifully here but its tough to say what she would do. Her mood is certainly very fervid and pumped up, they had been battling for quite a long time.
I'm def thinking that she wouldn't have the heart to kill even her deepest enemies in this sort of situation? She might toil with her own emotions about what to do but ultimately decide she just couldn't do it... As ppl have said though it is perhaps impossible to say.
If this were about Paine I'd quickly accept she'd act mercilessly as that seems to be her nature but with Rikku, for example, I also couldn't imagine her having the heart to kill someone who pleaded with her for mercy.