Kirun
Buzzfeed Editor
Do not neglect upgrading skills, they can make a huge difference, this goes for all units you're using.
In addition, you need to be going for skill "break-points" when skilling up. If you need to skill an operator up, don't just dump a point or two into their skill. Skill them up only if you can reach a break-point, as it isn't worth investing the resources otherwise. Skills get a big boost at skill levels 4, 7, and of course 7x3(these take a massive resource investment and should be saved for your top operators - they also can't be done until you get a training room in the base).
EDIT: Forgot to add that when you skill up an operator, you skill up all of their skills at the same time. Skills are meant to be a utility and serve different functions for different stages. An operator's E1 or E2 skill isn't always necessarily better than their base skill.