。
Trying to charge into the primeval forest alone — how brave of you!
Literal
Alone [with-で] primeval-forest [into-へ] charge-in [try-to-しようとは] you [topic-は] what [exclamatory-なんて] brave [copula-んだ].
An exclamatory sentence expressing surprise at someone's boldness. 原始林 (げんしりん, 'primeval forest') is a literary compound. 乗り込もう is the volitional of 乗り込む ('to charge into / to ride into'). ~とは marks the preceding clause as something surprising or remarkable. なんて~んだ is the exclamatory pattern 'how X!' — literally 'what kind of X it is.' 君 (きみ) suggests the speaker is slightly senior or equal, not a stranger.