Adjective to describe someone who is knowledgeable, resolute, and calm
Solution 1:
sage
wise, judicious, or prudent: sage advice.
http://www.thefreedictionary.com/sage
Solution 2:
The alliterative phrase 'cool, calm and collected' fits quite well, but if you want a single word, perhaps equanimous fits the bill. It's the adjective form of equanimity:
calmness and composure, especially in a difficult situation.
Samuel Johnson defined it as:
evenness of mind, neither elated nor depressed