The Prestige Achievement
Complete a Nightfall strike on Grandmaster difficulty.
Prestige difficulty can be selected from the Director when launching the Nightfall or Raid. The Power Level recommendation is 300, and you can not change your loadout after entering the activity. The timer on the Nightfall is shortened from the regular version, but other than that, it is largely the same activity, just harder.
Since this activity requires the highest Power level, I figured here would be a good place to link a guide to leveling. That guide also has links to two other guides at the bottom if you're still confused in any way. Between the three you should easily be able to get your Power level maxed out.