I think the most common thing students struggle with is math. The math that I do in the 2 courses I teach is very simple. Economics by old economists like Adam Smith was all words and no math but it was really hard to understand, and at some point people realized that math was a very useful way to communicate economic concepts. So now, much of economics relies on applied math in addition to data and empirical methods. For us it's really important to translate ideas and words into math, back and forth. Creating that bridge, between the idea and the actual problem on the page, is a valuable skill if you want to do any analytical work.
 

Practice as much as possible. When you write down an algebraic problem, try to explain it to yourself in words. What is the problem saying? And why does that make sense? Try to make sense of the letters on your page and the concept behind it."