Well, there shouldn't be much difference between these types and only partially underlined except that they are more of a pain to deal with as you have no 'correct, non-underlined clue' to help you out, and they require more reading.

The strategy though remains roughly similar. Read the original sentence; if you find a grammatical mistake, eliminate A and all others containing the same mistake. Then deal with stylistic issues and sentence construction. This is where they love to get you on those fully underlined ones because it is easy to play around with the whole sentence's structure.

If you do not see an immediate mistake, then you must read vertically and try to notice the differences among the choices - the differences should inform you as to what you are dealing with. Once you have spotted such differences (work one difference at a time) then you need to test each sentence for its own merit.

Remember- think grammatically first and only then worry about stylistic issues such as passives, redundancies, etc.

duongthang, I was in the same boat as you are. But now I feel more confident in solving such sentences. In fact after getting recommendation from this forum itself to try out e-GMAT, I purchased the course and even though I am through half of the course, my accuracy has improved. I used to be scared of long underlined sentences but not any more. And as Isaac has stated, the approach really is not any different from the sentences in which only part of the sentence is underlined.