The Boiling Lizard is a huge salamander that makes its nest in B3 of the Golden Lair. It is also the boss of said labyrinth, and can be quite a challenge, if you don't take a few precautions prior to engaging it:
First, the thing is no wimp. It has 21,300 HP, though you can cut its life energy in half by destroying the large cluster of scales in the back of the chamber you encounter it in. It'll give chase as you make your way around the room, so you'll have to dodge or destroy a plethora of smaller scales while avoiding it. There's a second entrance to the room along the western wall; use it to duck in and out of the room as necessary. The Boiling Lizard will return to the center whenever you aren't present.
If you manage to destroy the scale cluster, you'll not only cause the room temperature to drop dramatically, thus turning any remaining scales down to benign, red shards, but also force the Boiling Lizard to come to a standstill. This is a great time to get into vantage point and approach the monster from behind (giving you a chance at a preemptive strike).
As for equipment, you'll want to bring along anything with an affinity for fire. Fire Charms that cut damage received by elemental attacks in half are recommended for all party members. Icy weapons will help, too, although I've found it easier to inflict larger amounts of damage using non-physical attacks: Ice Rune spells that hit multiple times are incredibly effective against the reptile, as are freezing link skills. It's important to keep the damage consistent, because once its HP reaches about 10% it will begin recovering roughly 400 HP at the end of each turn.
The Boiling Lizard may also ocassionally shed a few of its scales. Definitely take these out quickly using any extra ice stakes or non-targetting ice spells, otherwise you'll not only take more recoil/fire damage from the walls the lizard spews, but it'll resist more.
f you want the conditional drop (Effulgent Horn) you've gotta slay it with a fire spell.