Skylanders Giants

Tree Rex's soul gem can be found on chapter 12 Molekin Mountain. In the section where you have to push the blocks and push the button to move on you will need to push all the buttons opening the next area you drop down to. BEFORE dropping down there will be a block you can push left or right, push it left and off the edge. Then drop down. There will be two blocks on top of each other, push them to the right. Go up the ramp and then push the top block into the gap. You can then go across and grab Tree Rex's gem.

Jet Vac's gem is in Chapter 14 Autogiro Adventure. When you are flying in the copter there are various platforms with arrows above them. Each one has treasure, hats, or the gem available. In the section called Caverns, if you land on the the second landing pad through the third door you will find the soul gem. (Credit to MC Evil Dead)

Collect both of these and the achievement should unlock.

If you have already completed these levels in story mode and missed the gems then click start and chapter select to replay the level.