Usually, we say“压力大” or “负担重”，

“好＋adj.”usually used in oral language, in written we prefer "很＋adj."to describe degree.

“帮个大忙” just like "give (me) a big favor" "v+adj.+obj.“

“帮”is the verb here, so no need to indicate the meaning of "give", and it's like a idiom, so cannot be translate word by word. "a" is “个” here means “一个”，“大忙”means "big favour" here, but "忙" alone doesn't mear favour, "帮忙"means "favour".

Also"忙"is a tricky word in Chinese, usually means "busy" as an adjective, but may infer different meaning in different areas in China, even they all speak Mandarin.

So, here the easiest way to remember is just use "帮个大忙" and consider it's an idiom.