'Mega Rocket' is one of the rotating game modes under Social > Arena.
In this game mode you can score points for your team by securing the rocket capture zone.
You obtain
Rocket Time when you're inside the capture zone securing the rocket for your team, kind of like a King Of The Hill mode.
I'm under the assumption that ANY sort of time spent inside the capture zone will net you progress towards this achievement. It doesn't matter who's inside the capture zone. Progress counts when either the 'Capturing' or 'Contesting' text appears on screen.