What are some tips for the Multiple Choice portions of the Writing section?

This section is very easy once you get a lot of practice because they test the same grammar rules over and over again. I won't go into all of them here but overall they like to include sentences that colloquially sound right but when you analyze grammar rules such as parallelism, singular/plural consistency, etc., they are incorrect.

Explain the mechanism behind contraction of a skeletal muscle.

A skeletal muscle (be careful, smooth muscles and cardiac muscles are different) is composed of thin and thick filaments, called actin and myosin respectively. The myosin has "myosin heads" which bind onto the actin and perform a "Powerstroke", pulling the actin toward the myosin center. Multiple units of this result in an overall contraction of the muscle.

How do I know which method to approach a set of differential equations?

Each method has key characteristics to look for. In integration, for example, if an integrand (the function to be integrated) is composed of an exponential term and a linear term, the ideal method to try is integration by parts.