Facing any kind of danger, our mind enters the so called fight or flight state, but maybe it would be more appropriate to call it fight if you can, flee if you can't, as even though you can always flee in some way even if it is from just a small rodent, or something intangible, such as a noise, the option to fight only really appears if your brain thinks it has properly estimated the danger and thinks it has good odds of survival if you choose to fight.
The situation Agi finds himself in, here, is quite peculiar in that sense. The option to fight is absolutely out of the question. There are too many unknowns literally everywhere around him. The risk of death if he so chose to confront the danger is at the very least extremely high and maybe even guaranteed, whether it be the creature he saw dead, the one who killed it or anything else in this damn transformed forest. Furthermore, he has never been much of a fighter, and never learned anything related, no self defense, no martial arts, no weapon mastery, no nothing, he has never willingly or unwillingly been in a fight, taken a punch or a kick ever, not even in middle and high school. And if he needed anything else to convince himself, he wasn't equipped for hostile confrontation either, with his cooking knife and dull machete.
Flight then, is the only option. However, in and of itself, it is impossible to flee, as there is nowhere to flee to. If he were to believe the weird messages that preceded, and came after the whole shift in the environment, he was alone in this acclimatization. No people means no help, no civilization, and really nowhere safe at all.
The system itself never really said whether or not there were any safe spots nonetheless, magically created like this whole biodiversity, however, from the options he chose and the risks that came with constantly being on the move, he barred that out too.
He will have to create his own safe haven, if that is really possible, and wait out the duration of the acclimatization without ever meeting face to face with any of the creatures roaming out there, hopefully.
Some important issues are to tackle then when it comes to playing the long game, and the first one is the acquisition of food and drinks. Agi heavily doubts that all this set up was meant to last a poor little week, which is about how much food he has got left, as for water he has a functional filter and therefore just needs to find a source. The optimal scenario would therefore be to be able to settle near a source of water, as food can be found pretty much anywhere thanks to it being spring and various berry bushes were popping up right left and center. He has never hunted anything before so processing meat isn't an option here.
However, this also gives another pretty big down with this plan. Even if he isn't constantly on the move, he will still be moving around quite a bit, be it out in the open to collect water, or in the dense forest for foraging. As much as he doesn't want to admit it, sooner or later, he might be brought to have some kind of confrontation with one of these creatures, and since he cannot really prepare himself physically, he might as well do so mentally, as for now that idea absolutely petrifies him.
To boost his confidence on the short term though, he can tell himself that he is certainly one of the best people on earth for the job, after all, he has experience in survival in nature (without any dangerous creatures around), and since he has been in this environment for a while already he knows the basic herbs and berries he can live off of. He is also decently fit, and has read discourse on the method by Descartes.
One could say that his odds are only second to a trained soldier armed to the teeth and with extensive survival and pragmatic knowledge, or people fitter, better equipped, more knowledgeable, with more experience, or with a better sense of orientation.
But besides from that, it is time for Agi to tackle the first problem he is going to encounter (unless he runs into bloodthirsty monsters before). And that is, a place to sleep. Indeed, in such a presumably hostile environment, where most living things could potentially kill you, where could you possibly fall unconscious for hours on end and wake up to tell the tale, and do so repeatedly too.
For that, there weren't that many options. He couldn't dig himself a den, nor build a secure home, so it was either on the ground or up a tree. And even though there was no guarantee that a tree was safe, especially at night, with potential snakes, spiders, felines or even primates, but that was a world's difference between what was going on down there.
He was pretty sure that the species he found lacerated on the ground wasn't one to climb up trees due to its sheer massive size, and he could only hope the one who left it in that state couldn't either. But weighing out between the two options was also very simple if taking into account this logical reasoning: what is up a tree can go on the ground, what is on the ground can potentially not have access to what is up a tree.
The tree he was on was certainly good for being perched up for a bit but when it came to sleeping in it, it was clearly lackluster, so he would have to go back on the ground to look for a better place for tonight.
Before that though, he would have to take on another important issue at hand, and that is navigation. His phone, gps and compass were all in the gutter, and his already challenged sense of direction wouldn't fare well in this environment. He needed to clearly mark down places and directions once he had a tree to stay at in order to know where to return to and not take his heavy backpack with all his gear every time he went to fetch berries or water.
His machete was pretty much already blunt, and his knife would also dull really quickly if he started to use it on trees, and he would need it for cooking and in the worst case scenario, self defense.
He still tried though just in case to leave a mark on the bark of the trunk right next to him with the machete, but he couldn't even make a dent.
But if you can't take something from the landscape, then add something to it. Yes it was spring and bushes thrived on the ground while leafs reigned atop of trees, but trunks albeit with some moss at times, were mostly free from greenery. He could therefore just take some leafy vines that could be found all around and generously wrap them around tree trunks of interest, and hopefully that would prevent him from getting lost.
Now all that was left to do was to set everything in motion. And for that, Agi decided to first go look for some kind of water source, and from there look for a nice thick tree on which he could sleep nearby. Berry bushes could be found more or less everywhere so he wasn't yet worried about this side of the spectrum.
And there he was, down on the ground again, flinching at every noise that seemed a bit too close for comfort, walking in a random direction and hoping for signs of a river or a small lake.
And it didn't take long before he noticed an increase in humidity and a slight downhill incline of the terrain, soon he could hear the flow, and seconds later, see it.
Before all this acclimatization nonsense, he was on a mountain plateau, and there were no known bodies of water taking their source up there, which increases the surprise of seeing such a large one right in front of his eyes. It was a good 4 meters wide and its color wasn't terrible either. He was confident his filter would be up to the task to make it drinkable.
He still didn't go to the riverbed though, as that area was quite open and he didn't quite want to expose himself to any kind of surrounding eyes.
The next thing on the list was therefore to find a tree on which he could sleep. For that he remained in a certain radius around the river in order not to lose it.
The task was surprisingly easy, it seems all the trees have beefed up in the forest and he had several candidates to choose from.
He picked the one with the best visibility to sneakiness ratio and stuck a vine around its trunk. There was still quite a bit of time before sunset, and he couldn't stay motionless for too long. He left his backpack up the tree, only taking his water filter, knife, machete, and a snack before going back towards the river to fill the filter up.
Once only a dozen meters away, the attention he put on scouting the surroundings doubled, every noise, strange hue, and smell was analyzed until deemed harmless by his brain.
Noise was getting muffled and smell covered up by the river, leaving pretty much only his eyes for first line of defense.
Creeping closer and closer he saw no signs of animal life, and at the line between the dense forest and clear river, which left him around three meters of solid soil to walk on before getting there, there was no room for being sneaky anymore, he ran down low bottle open, filled it up in one swift movement against the current and ran back into the forest, not stopping after the line but keeping the pace up until it felt necessary.
Nothing came running after him and he finally sighed with relief, getting back to his tree, he let the filter do the work while he pondered on his next course of action.
In the end, he decided to mark a certain perimeter around his tree. To do that he decided to keep wrapping vines around the trunks, every time he could only barely make out the vine from his base tree, and after running the vine a couple times around the tree trunk, he would tie a random but visible knot in the direction of his home tree.
He went ahead and did one full loop around his home, and even started a second one in the same fashion, go to the edge of being able to discern the last marked tree, and tie a knot in that direction, the most important was to never make mistakes and tie knots that would end up looping around pointing at each other because that would lose him. After that he went back to his makeshift house and mentally prepared himself for his first very long night in this unfamiliar world.