I'm struggling to capture any death Manticores, I cant actually kill any so I cant add them to the bestiary...

i have 4 characters at level 99, fighter, red mage, white mage and black mage but the Death manticores ALLWAYS run away before they can attack.
Have I leveled up to much, are you going to tell me I have to start again, because it's not worth it.

Anyone know how I can get the edge on these creatures?