If you remember the journey to Barba, then you'll probably remember this desert. In fact, you may have even seen the ruins where this battle starts if you're playing through a second time. Upon galloping (because you will need Agro for this fight) into these ruins, Phalanx will suddenly rise out of the sand and begin flying about. The first thing you notice about Phalanx is his size: This Colossus is huge, quite possibly the largest of the sixteen. The nice thing about this colossus, however, is that he won't attack you at all and barely struggles during the fight, even when you're stabbing his weakspots.
Start off by equipping the bow and trying to get underneath Phalanx (or at least pretty close). Fire an arrow at each of the white air sacks to deflate them. You'll hear them pop and retract to his body when you've punctured one. To make things easier, hold L1 or whichever button centres the camera on the Colossus, and if you have the bow equipped, the crosshair for the arrow will stay hovering on the Colossus. If you fire and arrow now, it will almost always hit an air sack. Once all three are empty, Phalanx will drop closer to the ground and lower the four wings located near his head. They'll skirt along the ground and kick up a lot of sand, so don't approach him from behind at this point. Instead try to cut across areas when he turns so you can ride alongside these wings. When you're close enough, press triangle to leap from Agro and press the grab button to latch on to one of the ridges on the wing. Don't worry if you don't make it on the first pass, as Phalanx seems to stay this way for quite awhile.
Once you've got ahold of him, climb up the ridges on them, and near the top, hang on until the wings are raised back to a horizontal position. Run along the top and jump onto his main body. There are three weakpoints along his back, each one behind a large spine. Find the first, crouch and charge-stab a few times until the symbol disappears. Once you've taken out one symbol, Phalanx will likely get agitated and close off the symbols (all of them) with a small flap over each. This is your clue that he's about to dive underground again, so jump off when he's fairly close to the ground and find Agro. Repeat two more times and your battle is over.
- This boss is no different on Normal or Hard Modes