While playing through the storyline, you'll come across a Spirit Tree (of some sort) that is the boss of the level. Inside, the tree holds Halis hostage. You'll be given little hints to watch his patterns, but I'm going to tell you what the pattern is.
First, he will throw his hands into the ground and his roots will spike up out of the ground. Needless to say, avoid getting hit. At the same time, four exploding spike plants. Change to Pummel Weed and use Creeper (LT + RT to go underground, then LT) to get underneath it. Move forwards towards the tree and use Uprooter (RT) to send it flying and exploding into his face.
He'll sputter for a second or two, then start shaking. This shaking will cause trolls to fly out of his branches like dandruff. When he starts to shake, transform into Rubble. Use your Rock Blast (LT + RT) to knock all of the trolls out for a little bit. The tree will then begin to laugh. This is your chance. When his mouth is open, use Rubble's Chip Shot (LT) and start hitting rocks into his mouth. After each successful swallow, the tree will begin to choke and take damage.
5-6 rocks in his mouth should be enough to kill him. If you don't kill him in the first set, he'll begin again roots from the ground. Just repeat the steps and you will kill him on the second go round. He's not that hard.
After the tree falls, you'll have rescued Halis and unlocked this achievement. Halis will begin to tell you a story about your father after you rescue him and will give you the Elemental Spirit of Ash. This, in turn, will also unlock the achievement "Found Ash" automatically.