僕は、何としても君の助けが必要なんだ。

、。

I absolutely need your help, no matter what.

Literal

I [topic-は], no-matter-what your help [subject-が] necessary [explanatory-なんだ].

何としても ('no matter what, by any means') is an emphatic expression of determination. 必要だ ('to be necessary') takes が to mark the needed thing as the grammatical subject — 助けが必要 = 'the help is necessary.' The なんだ at the end frames this as an urgent confession.