This youtube vid explains a lot about diabolos in the pinned comment:
/watch?v=lQ_K9x-6zNc
just, only use her if you've pulled her but don't have a 100% eidolon of your own. you also shouldn't use her in longer fights cause you're still using 2 off element characters
she's just a means to higher damage for those without a 100% eidolon