While exploring the Westersand you may have encountered a small area in the top right corner of the map that you can't traverse because of a terrible sandstorm. You might also have caught a glimpse of a huge Tyrant in the storm. After receiving the Dawn Shard you can do something about the beast.

The first thing to do is find Rimzat at Rabanastre's Westgate. He's there to study sandstorms but the storms are too violent for him to study. He then asks you for help. Now to the Southern Plaza and find Cotze who's sitting on the fountain. He directs you to Northon who's in the Number 5 Storehouse in Lowtown. Northon tells you that you'll need a Windvane to get through the storms but the one he used to have was broken into two pieces.

He sends you to the sectioned off part of Windtrace Dunes with all the Dynast-Cactoids. Search all of the cactoids until you find the Wind Globe, Northon's piece of the Windvane. Now head back to Westgate to find Northon, Cotze, and Rimzat all together. They put the pieces together and give you a complete Windvane. You now have the tool (other than battle prowess) to defeat the Earth Tyrant in the Wyrm's Nest.

With most boss and mark battles where the mark is large we recommend a triangle attack. Space all three party members in a triangle around the boss. This way your party doesn't get devastated all at once. This boss can and should be Slowed. If you have Decoy one technique is to have your hardiest character decoyed while your other two party members heal and throw range attacks. This can still be done without Decoy but there's no guarantee the boss won't change targets away from your toughest party member. In this case it's almost better just to have three range attackers to keep your party as spread out as possible. The boss's defense raises at the end of the battle so it's a perfect time to Quicken the tyrant to death.

Now, you will note the boss doesn't drop any items and you don't really get a reward from the trio back in town (who've relocated to the fountain by the way). However, it is not a profitless battle. Montblanc has your reward at Clan Centurio. You receive 1200 gil for your efforts.