There are two difficulties/modes that you can select. I suggest picking Normal mode.
Normal Mode -
You need to hit 15 targets in 20 sec.
Challenge Mode -
You need to hit 20 targets in 25 sec.
In this mode you need to be careful as if you hit claptrap it will fail.
If you hit Handsome Jack destroys all active targets, giving you the points for them.
You need to hit all targets or else you need to restart.
I suggest using an SMG and set it to Semi-auto.
Have any other suggestions to make it easier? Leave a comment.