I would recommend reading a good grammar book. The GMAT club Grammar book
is an excellent free option. It is tough to do well on SC with just one reading of the MGMAT SC
guide. Because it contains so much important stuff it can be a little overwhelming and hard to remember. Try to read it chapter wise, and do one chapter every day (should take around an hour). Spend half an hour to an hour reading what was discussed in that chapter in your grammar book or the internet (excellent resources available on the net). I have read the MGMAT SC
guide 4 times and can now get 85+% accuracy on the SC. I got 87% accuracy in the last 30 SC questions on the OG13 but i did not bother doing them until i had finished the SC guide at least twice.
If nothing works, then i would suggest taking online classes for SC or going for a tutor.
"When you want to succeed as bad as you want to breathe, then you’ll be successful.” - Eric Thomas