Reverse Groups challenge at CodeEval.com
It's pretty simple task and I think code does its job well. I would point couple minor style issues:


You don't need to use objectAtIndex method to access elements inside an array. elementsArray[i] is enough and it's more readable.
You can access count property using dot-notation - array.count. Again, it's more readable.
Combining result string from array looks cumbersome and complicated. And rely on description property. Why don't you create result string as you go without having to create temp array first?
You are dividing line into lineArray. Then you're accessing first and last element using lastObject and objectAtIndex:0. I would prefer to use either [0] and [1] or lastObject and firstObject.
Pass the code through standard Xcode formatter to clean up spacing and indentation.