Hi SDW2.
To get to Q50, for each topic that you're not strong in, you need to
- make sure you know all the relevant concepts for that topic
- learn strategies for answering questions involving that topic
- answer questions involving that topic UNTIMED until you
achieve close to 100 percent accuracy
and
get them correct in streaks of 20 or more in a row
- continue practicing questions involving that topic to maintain close to 100 percent accuracy while reducing the time per question to 2 minutes
Achieving the streaks of 20 in a row is super important. If you aren't able to achieve those streaks, then you don't really have the topic under control.
For some insights into how to achieve high accuracy and long streaks of correct answers, see this post.
Improving Your Accuracy on the GMAT