How should juniors present them self, when the situation comes to takes critical situation? No seniors supports are available to guide him immediately

Juniors are at the for front of the working field, they usually are left to handle things themselves however if they can learn to analyze the situation and consider all options, even taking losses into account, they can gain experience for that issue without the guidance of a higher individual and offered guidance. I think simulations of difficult situations should become more regular to juniors. 

Pro active is the answer, when it comes to a critical situation, what junior need to do is to be come pro active, flexible to try all possibilities and most important is to avoid desperate to report that he/she is not able to solve this situation. Hard and critical situation mainly distinguishing between the one who become responsible and accountable and a follower who always just do what leader tell him to do.

Juniors are important too and they present themselves well too in critical situations.  they must view the situation with practical action through proper focusing on the problem with eradicating any negativity and logical thought to achieve the outcome in the shortest time positively.  Asking the proper questions and  looking into past actions from records if any, taking steps in tis direction and doing the right thing recording the situation and steps taken to achieve the outcome.  keeping an alternative plan is good always.