However terrifying the mighty dragon appeared...
"However" + adjective/adverb creates a concessive clause meaning "no matter how." "No matter terrifying" is missing the word "how," and "Although terrifyingly" incorrectly uses an adverb when the linking verb "appeared" requires an adjective.
Whatever the massive creature did...
"Whatever" acts as the object of the verb "did" in this concessive clause. "However" would leave the verb "did" without an object, and "No matter" requires "what" to be grammatically complete.
And even if he had brought a real weapon...
"Even if" introduces a hypothetical or unreal concessive condition (he did not bring a real weapon, so we are imagining a contrary-to-fact scenario). "Even though" is only used for factual situations that actually happened.