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