Nabbers are creatures that capture a safka and you need to chase them down and corner them to free the safka. For this achievement you'll want to grab the nabber right as ilo and milo meet up, thus ending the level. They first appear in Chapter 3 in the first level, "Cut and Paste". This achievement can easily be obtained in this level. You'll want to start by moving ilo down the spiral all the way to where the Nabber is. Use the first cube to cross the gap,then collect the second cube and use them to bridge the large gap. Now, switch to milo and bring him down his path. Grab the flying block with milo and place it in the hole behind it and stand on it to fly over to where the nabber and ilo are. Now, all you need to do is corner the Nabber. Use ilo to block off a pathway then use milo to chase the nabber towards the dead end. The nabber should now be up against ilo and just go on top of him to "defeat" him and meet up with ilo in the same turn.