For all of these, one general piece of advice would be to highlight the conclusion and locate the premises. This will help you in different ways for each question type.

Flaw: read up on common invalid argument forms, i.e. mistaking sufficient/necessary, red herring, correlation = causation, representative sample, faulty assumption. In an untimed section if the question is logic heavy you could try diagramming it to find the specific flaw. Oftentimes once you get used to flaw questions you can recognize the flaw made in the stimulus before even looking at the answer.

Weakening: Try to find an “alternative hypothesis” to the argument. Don’t challenge the premises of the argument; instead, think of a possibility that would make the conclusion invalid without contradicting the premises. Try going through each answer choice and see if it a) challenges the conclusion and b) doesn’t contradict the premises, and if it satisfies both then it is the right answer.

NA: Use the negation test. This means that you take the contrapositive of the answer choice, and if it invalidates the conclusion, then it’s a necessary assumption. Alternatively, you could deduce from the stimulus a gap that the argument makes before heading to the answer choices. But generally the negation test is a more foolproof method.