Hi eitijan.
Basically, in order to become more effective in handling GMAT sentence correction and critical reason you have to go beyond learning rules, tips and strategies to training yourself to see key details and the logic of what is going on in the questions. In other words, what you have done so far is a good start, but it's just a start.
Here's the type of thing I see going on.
A person gets the impression that sentence correction is about rules and grammar or that getting CR questions right is about learning some key strategies, and the person learns those types of things and reads lots of explanations, figuring that by learning enough he can master those question types. The thing is that, contrary to the types of things people often say, those questions are not really doing things like "testing parallelism". Those questions use things like parallelism to test vision, skill in using logic, and decision making skills.
So in order to get more right answers to those types of questions you need to work on using vision, skill in using logic, and decision making skills.
What that means is that you have to do practice questions slowly, analyzing every prompt, question and answer choice to see what is going on. You need to get used to figuring out exactly why the wrong answers are wrong and the right answers are right, and you need to become good at doing this yourself, without looking at explanations. Yes, explanations can be helpful, but over time you have to get to a point such that you are using explanations less and less and creating your own explanations more and more. In doing that you will develop an eye for the key details and skill in using logic to arrive at the correct answers.
While doing questions in this manner, you have to not worry about time much in the beginning, focusing more on getting a high percentage of right answers. You can be sure that if you can get most practice questions right, taking 15 minutes per question, you can learn to speed up while continuing to achieve a high hit rate. So at first work on hit rate and then work on doing questions in less time.
