150 points: $250
250 points: 400 XP
350 points: $500
There are tons of Time Barrels scattered about, so it's best to do a few practice runs and get the locations of all barrels down before giving a serious run a try.
After 90 seconds, heavy gunners will spawn and attack you. They will continue to spawn until the end of the Trial. This is both a detriment and a plus for you; killing these guys will earn you tons of points, but they're super resilient, too. Try taking them down with repeated grenade tosses.