When it comes to things that could cause her harm, Elsa tends to be overconfident. In the past her abilities have protected her without her putting any thought into it. This has given her the impression that she will be able to manage most altercations she could find herself in.

However Elsa has no idea how to harness her power for a fight, defensive or offensive. If she is not fearful or upset, she will not be as good at defending herself. For Elsa’s powers to really work in her favor she has to be feeling an intense emotion.

In the case of an actual confrontation, fear or anger would work best, but these are the feelings Elsa tries to rely on the least. Ultimately making her weak if she is indecisive about what she hopes to accomplish by using her abilities to harm another.

This is basically to say that the more Elsa is thinking about her actions, the worse she is in combat.