Shiva is very easy in comparison to other Eidolon battles, once you know what to do. In this fight, you'll have unlimited health because Stiria (the whiter one) will constantly heal you. First off, you'll probably want to start off with a Commando. When the attacking sister begins to charge her ATB gauge (it'll say something like ATB charge), that's your cue to attack for a few more seconds, then quickly switch to Sentinal. Guard for the remainder of her attacks. I suggest switching between the two at appropriate times to speed up the process, but you can stay Sentinal the whole entire fight and still make it in time. That's what I did. Still, it'll get pretty close, so it could take you another try to get used to the fighting style.