“Make your way past the Vex defenses to reach the Endless Steps.”
Make your way forward from Campus 9 to the Endless Steps. Taken are now entering the fray. Be warned: it is very important for the whole fireteam to stay together here. Clear out the Taken Portal in front of you and whatever came out it (likely Taken Goblins).
With the first threat dealt with, you need to find the Arc Energy. Once again, it can appear in a number of places. It does seem to favor appearing on the Steps themselves. Expect to deal with some cross-fire between the Vex and the Taken. Grab the Arc energy and it is time to get it to the final barrier in your way.