Kolima forest is not a long walk away from Kolima. Head northwest and you'll see on the map a forest with a gold entrance to it. Go up to it and prepare to enter Kolima forest. The enemies in Kolima forest will start to become more plentiful and a little more difficult than usual. They're still nothing to be afraid of, but make sure that you don't run out of PP on Isaac and get left without a heal.
Head due north to get to the first easy puzzle of Kolima forest. Head northeast and go across the river using the log path that's already there. Head west, push the log that's in your way west and then go around it to the north. Keep going west until you get to the next screen. Here you want to head south and then west, pushing the log in your way to the left. Keep heading north along that trail until the path pulls a U-turn that will take you to the next stage. On the next screen, hang to the right and go up. Push the vertical log towards the west, then head north around all the stumps. Push the horizontal log down, then the vertical log east, then go down to the log on the bottom and push that one down. Push the horizontal log above it back up, and then push the vertical log all the way back to the west. This will place the log in the water, so jump across it. Head up the stairs and to the east to get to the next stage. This next screen is a little tricky, but at least there aren't any monsters on it. Go east until you see a switch that a sign says do not touch. Obviously you have to switch that, so press A when you're next to it to lower the water level. Now, head down the stairs and push the topmost vertical log to the left. Go back to the switch and hit it again to fill up the water level, then hop across the logs to get to the ledge on the other side.
On the next screen you will find Tret and his lady counterpart. Head over to the east to find Tret, and climb the vine on the front of him to enter in the door above his head. The main objective of the Tret "Dungeon" is to get to the top floor where you will then proceed to fall down on a leaf in the middle, and continue to fall through those open holes in the middle of each screen to reach the basement where Tret is. So, when you first enter the tree give it a nice save incase you die later on. Now, head to the right, then proceed north and then to the left. Climb the vine to reach the next floor. On this floor, head all the way to the bottom most part of the ledge. Now, jump across the three leaves on the bottom to reach the other side. Note: jumping back over a leaf you've already jumped on means falling down at least 1 floor. Head out the door on the right to end up in one of Tret's branches. Head east without deviating from the main branch to find a vine to climb up to. Do so, and then start heading back west. Once you're back in the tree, head around the tree clockwise, until you can reach an inner ledge. Note: You can jump for the Psy Crystal at the north part of the screen, but you really shouldn't need it. Jump to the leaf and continue north, as you don't especially need the treasure chest (it contains a healing ring that will restore 70 HP (thanks to Josh Moore for that correction) when equipped, although it gets worn out reasonably quickly.)
On the next floor, head out the bottom door and go east until you get to a branch which you should climb up. Follow the branch until you get to the main one, and here head left just a little and then up the first branch to the north. Here you'll find a Djinn that you're going to have to battle, so save and heal before you fight him. This guy is a little harder than usual, so make sure that you keep your health up using herbs and Isaac's Cure Psynergy. When you're in the battle, use Flint to his best abilities, also try to summon any elements that you have. This Djinn doesn't like to be captured, so unless you beat him substantially he will run. If he does run, head back to another screen and then back to where he was. He should show up again and you can give another shot at him. Once you defeat him you'll get the Jupiter Djinni Breeze. Head back into the tree using the main branch to get a treasure chest that has a nut in it. Then you'll want to intentionally drop down to the previous level. Now that you're on the previous floor, head counter clockwise and out the door on the left. Go off the first deviant to the south and up the vine. From here, go back east until you get back in the room. Here, go to the southern most part of the room and hop to the middle leaf. Once there, hop one to the right and then back to the middle leaf to fall down the center and into the basement.
Now that you're in the basement, it's time to fight Tret. Save before you do this and inactivate all your Djinn. Make sure you have full health, as well. Tret has about 350-450 HP, so it's going to take you a few rounds to get him. Your best bet is using Summons, then Flint with Isaac, and using Psynergy with the others. Tret's attacks aren't too great, so you shouldn't have too much of a health problem. Once he's dead Tret will come to, realizing that he shouldn't have turned people into trees. However, he no long has the strength to cure Kolima, so you need to begin a trek to get an item to heal him (more on that later.) Once Tret is done talking the talk, use retreat to get to the entrance of the tree, then use retreat again to end up back at the start of the forest.