Once You've complete Earth rhealm, visit it on an exoloration duty. use this time to find Chests and motherloads, also complete the four challenges in each area if you really want to. This creates more chests so it is worth it if you want to put the time in.
These guys are good to churn out a lot of the combat trophies if, again, you want to put the work in.
Running around this map will get you the trophy in an hour or two, as long as you've already started to make your way to the water rhealm, you should already be pretty much there.
You can track you progress in the menu, there a a merit directly linked to this trophy.