The speaker is using humorous language.
"In the grand scheme of things" means "the big picture" or "the whole world" or "all of history."
"I am stuck in traffic and I'm going to be late. It seems important to me. Oh, well, in the grand scheme of things it's nothing."
"Crushing this" is an informal phrase. It means accomplishing a task well. Figuratively, the task is like an enemy to be destroyed. "Crushing" the task means doing it so well that you have conquered or destroyed it.
"I've gotten an A on every chemistry test."
"Wow, you're really crushing it."
"Legitimately" (note spelling) really means correct, authentic, valid, or legal.
However, here the speaker is using it in an informal way. The speaker just means "really" or "truly."
Here's a more correct use:
"He won the Tour de France bicycle race."
"Was it a legitimate win?"
"No, he was using performance-enhancing drugs and they took his awards away."