I've recently purchased the Arz-307 7-String Ibanez guitar.

http://www.guitarcenter.com/Ibanez-ARZ307-7-string-Electric-Guitar-106566805-i1697611.gc?esid=arz-307

I Planned to Drop B tune it, so I purchased a pack of Ernie Ball 7-String Power Slinkies, which are 58-11s.

Yet, it doesn't stay in tune all that well. Any Suggestions of what to do to possibly have it stay in tune longer?

Still need help

Drop B is a dirty ass tuning - but you should look at the Zakk Wylde strings.

They are 0.60 on the low, and tbh, it's really fuckin low lol.

I use them for anything Drop C - but I think on a guitar with a locking nut, they'd work a charm.

TBH i have never needed anything lower than that, so i can't help below that. But I will vouch that the ZW's are beasts for pinches

Sure its not just the new strings stretching before they set better?

When you put the strings on, make sure they're wrapped around the tuning pegs neatly a few times. Pull on them a bit when they're freshly installed, then re-tune, and repeat - the tuning peg is the most likely place for a string to slip a bit, thus changing the tension a little bit. Once they're on and being played, there isn't too much you can do. Changes in the amount of tension on the strings wear them out more quickly - if they're slipping around the tuning pegs while you're not playing, that can contribute to poor tuning stability.

The other obvious setup aspects apply too - make sure the guitar is intonated and set up for this amount of tension.

I use Ernie Ball .064 or .066 for B and the tuning stability is alright.

Use D'Addario .12's