Simple strategy is what is it that you must take from the given options which should be 100% be true for the argument to hold. You will not be able to negate all the time. On higher difficulty questions, negation becomes very complex as well, so think of assumption questions as any other questions, you need to find what it is that the author has not said but is an absolute requirement for the argument to follow. Then when stuck in between two options try the negation only if required. Try it out and see how you feel. Also, make sure you understand the basic skill of breaking down the argument into the premise and the conclusion. Think of the space between the premise and the conclusion, you should operate in the that space with the options available, always find 4 wrong answers as opposed to 1 right answer.
Also, end of the day this is logical thinking, you will need to make sure that you do not want to fall for traps, meaning for answers that could be true but are not really required for the argument to hold. As the difficulty levels rise, the answer choices are very close, so you will need to train yourself to separate the traps from the right answer.
Could someone please add this new question on the Verbal forum, this is from practice exam 4.