Tlaloc is the third boss in the game. You'll fight him shortly after acquiring the Flame power for the Majin.
Fighting Tlaloc is a bit more straightforward than the second boss, B'alam. Another two-phase fight that can be handled as follows:
Command the Majin to follow you and walk out onto the sand. Tlaloc is slow and his fireball rain attack is easy to avoid, so don't worry much about being hit. Pick up a bomb and have the Majin use "Flame" to light it. Throw the lit bomb at Tlaloc (don't worry about how much time is left on the fuse). When hit with a lit bomb, the bomb will explode and the boss will fall on his side. Run up and attack his flashing red blowhole/spout. Repeat this procedure 2-3 times until a short cutscene is triggered.
Tlaloc will move around faster, but there's still not much cause for concern. Pick up a bomb, have the Majin use "Flame" to light it, and throw it at the boss. After he's hit, he will usually say, "Such humiliation!" and start sucking in sand. Quickly light another bomb and throw it in his mouth. The bomb will explode and he will slump down. Command the Majin to attack him. The Majin will open his mouth, allowing you to get inside (it sounds crazy, I know). Jump in Tlaloc's mouth and hit his red weak spot a dozen or so times. Jump out of his mouth (the Majin will warn you before it closes) and repeat the same steps 2-3 more times. You'll eventually be prompted to hit 'B' for a Combination finisher. Enjoy your cutscene and gamerscore.