As Osiris and Set fight each other, they throw off blue and gold energy bolts, which you should try to avoid. Meanwhile, 3 Marks of Set appear and begin spawning various enemies.
NOTE: While you run around lighting the beacons, you may want to check off the simple Challenge: Run Between Set's Legs at Least Once. See the main walkthrough for details.
There's nothing you can do to harm Set at this point. Before you can attack him, you'll first need to close the 3 Marks and ignite the gas jets left behind. The order in which you do this doesn't matter, but you must move fairly quickly since the Marks will reactivate if you take too long. As soon as you've closed one of the Marks...
...immediately use your torch to ignite the gas jet left behind.
Then quickly run/roll to the next Mark and do the same thing again.
Once the second beacon is lit, move around to the third. You can drop and detonate bombs to deter any enemies that follow, but the main thing is to destroy the Marks and light the beacons, since this will enable you to defeat Set.
Use the Staff to close the third Mark...
...and the torch to light the beacon. If you go too slowly the previous Marks will reactivate and you'll need to return to them and repeat the process.
The co-op game is basically the same, except SET is proportionally tougher, and there are more extra enemies at each stage. You might want to have some players concentrate on fighting while the others activate the beacons. Here, Horus and Carter light the beacons while Lara and Isis defend.
Once all three blue beacons are lit, OSIRIS immobilizes SET, and he becomes vulnerable. Put a little distance between you and him, select your most powerful weapon, take aim, and fire continuously to do as much damage as possible. The Onyx Rocket Launcher is the best weapon for the task, if you have unlocked it. Otherwise, use the regular Rocket Launcher.
When you've reduced Set's health bar by about a third...
...the first phase of the battle is over and the 3 Marks reappear.
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