>>39772122take mimikyu for example. he's a rare spawn in roofed forest biomes at dusk and dawn. you're incredibly unlikely to find him in that short amount of time, especially because all the trees will spawn tons of garbage like hoothoot leaving fewer possible spawns for land based pokemon. you can climb up into the sky and build a huge platform out of dirt or smooth stone though, and you'll be far enough away from the ground that nothing will spawn there, so you'll only get land based spawns and they'll all be conveniently right near you on the same level
desu though it's probably easier to burn down the forest
