1.He can show great power in crisis；
2.He can show great power at critical moments;
危急时刻可以是“in crisis”,“in the middle of crisis”,“at critical moments”,“at critical times”.怎么用都不会错到哪里去.
3.He can bring out his great potential in crisis;
show great power表现出很强的爆发力,bring out great potential发挥自己强大的潜力.两者的意思差不多.
tremendous当然是巨大的意思,而且是大得吓人,但常用词great同样能表达很大很强的意思,而且传达出一种欣赏赞许的意味,何乐而不为呢?
多说一句：break out不是及物动词短语,也就是说后面不可以接名词作宾语,不论是break out potential 还是have potential to break out都犯了这个错误,属于中式英语.