I think it's almost better to just buy a box and try to pull them but as others have said, if you need them, then just buy them. I'd much rather buy a few boxes of GR and get them that way while growing my collection.
My view on this, is:
If you're looking to compete, buy the singles.
I opened two boxes of Guardians Rising, and didn't get a Lele... so, now I have to spend the money on them anyways.
This was my decision, and I don't regret it, but I definitely wouldn't recommend it as an alternative.
The only reason I decided on the packs, was because there are a lot of valuable cards in the set.
Of course, If you're looking to collect—or just wanna open packs—go ahead and buy a box!~
Buying a box is also viable if you play PTCGO.