It depends person to person and what sort of score you are looking for.
Hardest to get 7 is writing. Many people struggle to get that score.
The rest is manageable.
For writing, just make sure to write a great letter for Task 1 in 20 minutes. I know IELTS experts suggest that more emphasis needs to be placed on task 2, but I felt from the feedback I got with my IELTS scores that scoring 8.5-9 in Task 1 is very crucial to get a writing score over 7.
See, the point is that no matter how well you think you do on task 2, getting 7 or more there is really hard as it takes exceptional English skills to get there. Getting the best possible score on task 1 helps you negate the score you lose on task 2. Task 1 is so much easier and straightforward as it is shorter and clearer on the goals of the task.
In my case, I took 20 minutes to write a very good letter, but I fell short on the number of words on task 2. I only wrote around 220 words and made a couple of spelling mistakes, and still managed to score a 7.5.
Also, make sure to work on your paraphrasing as your introduction paragraph for task 2 will mostly be a paraphrase of the question itself. It also helps to use different forms of the same word and possibly a few synonyms at various occupancies of the same word in your writing. Linking words / transition words are important too as they help structure you paragraphs better.