“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 says this because she is extremely pleased.
They did just a phenomenal job on this event.
When used in this way, "just" means "very" or "completely". It can be used with positive or negative adjectives:
This was just a horrible, horrible idea!
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.
"Phenomenal" means "really, really good". You can use it to talk about, for example, an amazing meal that you ate:
Susie, that was phenomenal. Thank you so much!
"Phenomenal" is not as common as other similar words like "great", "amazing", or "outstanding". So you can use it when you want to praise something in a way that sounds new and fresh.