Once you reach the area with the living trees, kill two fish.

When the mosquitoes spawn, they will head for the dead fish and then die once touching the water.

The mosquitoes will leave behind a red orb which restores your health, and will continue to respawn and repeat the same process until you have yourself a mountain of infinite HP, making the achivement 'Cautionist' alot easier to get.

Or you could like, just use it to your advantage..

..Whatever works. ¯\_(ツ)_/¯

Hope this guide helps!