If you're looking for a relatively modest way to make the campaign more difficult, try the Catch skull. It isn't as insanely difficult as trying to play with the Assassins skull active, and it doesn't interfere with your display like the Blind skull . This skull simply allows your (non-Flood) enemies to throw more grenades at you.
This skull requires that you play the Metropolis level—on Legendary—until you reach the mission objective entitled "Regroup with Marine forces in the city-center." The notification text for this skull is "Catch."
The Catch skull is located in a section of Metropolis called "This Town Ain't Big Enough for Both of Us." To get to this stage, you will need to take the Scorpion across a large bridge and then make your way through an underground highway. Near the end of the underground section, you will come to what appears to be a dead end for the Scorpion. Some marines will join you in a Warthog and one will offer you a shotgun. Despite how it may appear, it is possible to drive the Scorpion around the barrier. Driving up and around the barrier allows a player to keep the Scorpion for a little bit longer. This makes the next section much easier to complete.
After you exit from the underground highway, you come to a large open-air room filled with Jackal snipers, Grunts, and some Elites. Clear this area so that you aren't disturbed while retrieving the Catch skull. As usual, having the Envy skull will help you progress faster than normal.
Once you have defeated the enemy, move around the left side of the room and then onto the center platform (see ). You will see a large semicircular beam going from the ground to the top of two matching towers. Do a grenade jump at the bottom of this beam and walk all the way to the top.
Figure 1. Lining up the grenade jump for the Catch skull
Next, jump over to the ledge on the left side. Continue around the left side of the ledge and you will find the Catch skull's spawn spot (see ). Be careful not to fall down from this platform; you will not survive the fall. Now get ready to run like you've never run before; a shower of grenades awaits you around every corner!
Figure 2. Not just an Easter egg—a great view too