Here is the full list of all 18 Forest Guardian achievements.
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Die 20 times from the Frankenstein
Die 30 times from the plants
Shoot 200 times
Spend on level 10 for more than two minutes
Level one is started
To reach level 2
To reach level 3
To reach level 4
To reach level 5
To reach level 6
To reach level 7
To reach level 8
To reach level 9
To reach level 10
To reach level 11
To reach level 12