Structuring your essay
Like any essay, you need an introduction and a conclusion. You should start the introduction by restating the argument in some way and then by giving a broad, general statement about what is wrong with it.
There are several ways you can structure the essay, but in general the simpler the better. Thus, I recommend one of two ways. You can either make each paragraph describe a flaw and how to improve that flaw argument, or you could make each paragraph describe a flaw and then put the improvements in the conclusion. Regardless of which one you choose, pick one and stick to it.
The conclusion should be a quick summary and include the improvements or not, depending on what structure you chose. You should not repeat things you have already said verbatim. Find a new way to say something, even if you are technically repeating yourself.
Budgeting your time
Obviously, you should start the AWA by reading the prompt. This should take you two minutes max. When reading, make sure that you identify the premises and the conclusion. Then, you should brainstorm 3-4 flaws that you find in the argument. This should take you 3-4 minutes. You will use these flaws to form your outline.
The bulk of your time should be spent writing. Follow the structure that you decided worked the best for you and try to be persuasive in your writing. Make sure that you leave a few minutes at the end to read over your essay and correct any spelling or grammar mistakes.