The last part of the Taiga chapter requires you to reach the dam and meet with Alyosha.
Enter the told tunnel and chase spiders away with your flashlight. Open the door and go up. Your flashlight has stopped working. Run towards the torch and light it up to scare the spiders off. Move from one torch to another. Soon, you will lead the stairs.
Go up - you will be able to use your flashlight again. Go through the air duct. Use light to eliminate enemies.
Go to the generator and try to turn it on. Sadly, it needs fuel. The nearby canister is empty - go up the stairs and take the canister in the corner, near the locked door. Go down, activate the generator, and interact with the fuse box to proceed forward.
Follow the path and then go up the stairs. Use the ladder - you will meet Alyosha. Follow him through the devastated village. Soon, you will come across the mutated bear - you can't avoid this fight. Shoot the bear and throw Molotov cocktails at it. Artyom gets stunned - watch the cut-scene in which the bear will finally die.
The chapter's finale and Alyosha's fate
Watch the cut-scene featuring Olga and the locals. If you showed mercy, released prisoners and you didn't kill any pirates/Pioneers, Olga will let you go. Otherwise, you will have to run away and Alyosha will get heavily injured. From now on, he will move on a wheelchair.