How can I get the highest amount of marks for a 100-word answer? [of the writing paper]

Getting a high mark in one of those questions isn't difficult: once you are aware of what the examiners are looking for, it's just a matter of exam technique.

The first thing you have to do is read the question carefully and make sure you cover all points they are asking for. You will lose marks for any aspect of the question you haven't covered.

Plan what you're going to write before you do so: even if you spend two or three minutes planning it, once you have it on paper, writing it and developing will be much easier, and you will be much more focused. 

As to the answer itself, try to use different tenses: this will show examiners that you really master the language, and that you have the ability to adapt it to different scenarios. Another way you can achieve this is by using different sentence structures: don't start every sentence with the word "I", use connectives... Even if you make minor mistakes, examiners will like the fact that you thought outside the box!