これ以上ここでもたついてると、後ろの第二関門からも増援が来て、挟撃されるかも知れねーんだぜ。

、、。

If we keep dawdling here, reinforcements will come from the second checkpoint behind us, and we might get caught in a pincer attack.

Literal

This-more-than here [at-で] dawdling-[progressive-てると], behind [gen-の] second-checkpoint [from-からも] reinforcements [subj-が] come-[and-て], pincer-attack-[passive-される] might [explanation-んだぜ].

もたつく 'to dawdle / fumble' is a colloquial verb for wasting time through clumsiness. 挟撃 'pincer attack' is a military compound (挟 'pinch' + 撃 'strike'). The ending ~かも知れねーんだぜ stacks several casual-masculine markers: 知れない → 知れねー (vowel shift), んだ (explanatory), ぜ (rough masculine assertion). The whole line reads as urgent battlefield dialogue.