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You can use the cheat codes once you obtain the items,to do so though you have to go to your menu and click / tap on her head, and you can use these codes 

Player.RifleMagazine = (10 through 100k)

Player.PistolMagazine = (10 through 100k) 

C 0 ( paid version / 1.0.1 ver) 

Also to kill the plant, the branch that shoots the poison orbs is what you shoot during the boss fight, then just spray the second phase with rifle