To had access to brainstealer you need have 6lvl of protection "shield" and to got lvl6 shield you just need collect enough Crystal Splinters (points given by complete task in wz7-9) so to answer your question, NO, kill of magmoid is not needed for got access to brainstealer, if u want you can collect all needed spliters only doing task on wz7 just will take you longer to reach required amount of "points" to lvlup shield. Task in wz7 give 10splinter each task, in compare just magmoid give 40. So is your choice what task you want to make but any of them is "require".