Antonyms for the phrase "panic stricken" could be words that describe being calm or relaxed. Some suitable antonyms include composed, tranquil, poised, serene, placid, collected, cool-headed, and collected. These words describe someone who is capable of remaining calm and level-headed even in stressful situations, rather than experiencing an overwhelming sense of fear and anxiety. When someone is composed or tranquil, for example, they feel a peace that comes from within, which helps them to maintain a clear mind and make rational decisions. By contrast, those who are panic-stricken often feel confused, disoriented, and unable to think clearly.