“They did just a phenomenal job on this event.”

In a meeting for the entire staff, the Marketing Director is complimenting how well her employees put together an event & she is extremely pleased. She says:

They did just a phenomenal job on this event.

just a (adjective)(something) / (be) just (adjective)

When used in this way, "just" means "very" or "completely". It can be used with positive or negative adjectives:

That meal was just delicious.

This was just a horrible, horrible idea!

When you're using "just" in this way, don't stress the word "just". Stress the adjective that follows it:

They did just a phenomenal job.

do a good job on (something)

This means to do something, like an assignment or task, well.  You explain what the task was using "on___"

You can use other positive adjectives instead of "good":

They did a great job.

She's doing a fantastic job.

We're doing a pretty good job on this.