1 action per panel. You can only really do 1 action per panel.
For example, having a storm brewing and a forest being blown up by lightening.. is 2 actions.
Freezing and blowing away something is 2 actions..
Etc.
The letter is nice, but most people will skip it because it's an info drop and its very long.
Also, its OK to have panels that are just actions and no dialogue. Your comic is very dialogue heavy and I think for an action/fantasy comic, its too much. Let your character's expressions and actions speak for them.