So to start off you'll need to set up co-op play on the mission Regret (i think well is the mission after oracle)

Now heres how to get to the secret,

So when Cortana says "There all pouring out of the middle lets get in there" you go inside there w/ partner and keep going untill you see an infantry of grunts and jakals (before the waterfall) there you will see a ledge on the left with a chunk of rock there (the rock is a square), tell your partner to jump on the rock and crouch, you jump on him, then when you jump off tell your friend to stop crouching, then jump again. This may take a while but in a few tries you'll get there. Finally, tell your buddy to die or wait till he teleports to you. So you walk down the pass go right fall of and there will be a room with 2 grunts and 2 energy swords (full)

Hope it helps!!!!!!!