Not all colleges require you to take SAT Subejct tests. Most of the institutions require only SAT Reasoning test. However; you will also find some colleges that will accept a good score on the Chemistry SAT Subject Test as part of their science admissions requirement. In other words, if a school requires three years of high school science, it may be possible to take two years of science and do well on a science SAT II Subject Test in a third field.
You must prepare it if the school demand the score of SAT II Chemistry.
The level of the test is based on FSc first year course along with some topics from the second year course. Students of A Level can easily take it with some preparation.