Easy but takes a little time to do (depending on level).
Simply head to a warp pad:
Select headquarters
Choose the 2nd option (the shield training one)
Go kill a target dummy
I found many of the dummies health would hardly move so I ran around and found a room with one single
"Lvl 60 DPS target Dummy" (dummy was in yellow) his health seemed to go down faster. He's on the left side towards the back of the area.
Takes a while so you might want to rubber band the controller.