“严阵以待”常见的英文表达是 “be fully prepared/on full alert (to meet an enemy or deal with a situation)” 或 “stand ready in battle array”。
The troops are fully prepared on full alert to meet any possible attack.（部队严阵以待，准备应对任何可能的攻击。）
We should stand ready in battle array to face the upcoming challenges.（我们应该严阵以待，迎接即将到来的挑战 。 ）