While I cannot answer the question (which is obviously very complex) in full in this limited space, I can give you a number of ideas. First, the question requires that you argue for a balanced government or a situation where either the executive branch or Congress is preeminent. First, you will have to decide which to argue for, which is of course based on your personal opinion. You will then need to go further, pointing out the strengths and weaknesses of your position. So I will highlight some historical examples of the strengths and weaknesses of some of the scenarios for which you might argue.
Historically, a stronger executive branch emerges in times of extreme crisis, when quick decision-making was necessary. For example, during the Great Depression and World War II, Franklin Roosevelt exercised unprecedented power to deal with these successive challenges, which posed enormous and profound threats to the nation. In so doing, he permanently enlarged the executive branch, but his presidency also created the specter of abuse of presidential power, as when he attempted to enlarge the Supreme Court with justices friendly to his initiatives and when he ordered the internment of Japanese-Americans during World War II. Other examples of the abuses that can occur when the powers of the presidency go too far are the Watergate and Iran-Contra scandals during the Nixon and Reagan presidencies, respectively.
Arguments for a stronger Congress might focus in particular on the ability to wage war. Since World War II, no Congress has declared war, but a series of Presidents have nonetheless used their powers as commander-in-chief to wage war in such countries as Korea, Vietnam, Iraq, and Afghanistan. Many argue Congress should be more sparing in delegating this power to the President (by denying funding). On the other hand, many decry the ability of Congress, especially the Senate, to obstruct the President, as has been the case during much of the Obama administration. The current refusal of the Senate to hear the nomination of Merrick Garland to the Supreme Court is an example of this.