Lots of people approach me saying they need a 7 in IELTS. My advice is always the same: know what that means. It's easy to see what a 7 translates to in Reading and Listening but sometimes people don't know what a 7 means for speaking and writing. Google this: IELTS speaking band descriptors as well as IELTS writing band descriptors. The descriptors are available on PDF all over the Internet. Familiarize yourself with them and what they mean.
A 7 in writing means you will need to do lots of things. In terms of task response, first, you'll need to address all parts of the task. So, if there are two questions, you will answer those two questions. Advantages/ Disadvantages? Talk about them both. Your position will need to be clear throughout, so yes, say what your position is in your introduction. Lastly, back up your main ideas with support, support, support. An 8 tells us that you have to develop your response with EXTENDED support, so after every sentence you write ask yourself, "how, why, what, who?" If you can give an answer, then perhaps that answer should go in your essay.
Coherence and cohesion tell us that your essay needs to have a logical order & progression. Cohesive devices are necessary but they are more than just Firstly, Secondly, thirdly. Even pronouns and words like this and that can link your ideas together. So, use a variety. Another warning would be not to OVERUSE cohesive devices. So, sentences should be linked together but more seamlessly.
As for lexical resource, why use "big" when you can say enormous, monumental, substantial or vast??? You want to be as precise as possible. You could say, "air pollution is a big problem in our society," or you could say, "air pollution is a problem of monumental proportions in our society." And I know that spelling in English is no easy task but it counts, so do be careful.
Grammatical range means use more grammar than just Subject + Verb + Object. Use subordinate clauses. Use advanced grammar like inversion, subjunctive, participle clauses, conditionals, relative clauses. If you are not sure what these things are, open an advanced grammar book or Google them.
In an 8 essay, the majority of sentences are error-free. That is your goal.
If you keep in mind what an 8 is and have that as a goal, then you are more likely to get the 7 you actually need.