The Gaping Dragon is an easy boss, once you figure out its attack patterns. You can cut its tail off to acquire the Dragon King Greataxe, and it's pretty easy to cut off after it does its charge move. You can also summon (only if you're alive) Solaire (next to stairs leading to fog gate) and Lautrec (in area with Heavy Crossbow) if you need extra help. The Gaping Dragon has a few attacks that you should keep an eye out for. Be careful in front of it, as it has a move where it can grab you for some big damage if you get too close. It can also spew some acid, which can damage your equipment as well, so make sure you get far away from this attack. The two attacks you'll most likely see the most are its slam and its charge; it will stand up and then slam down, so make sure you don't stand too close. When it's standing on all six legs, it will crawl (charge) across the room. It is after these two attacks that it's most vulnerable, so get two to three hits in before backing up and waiting for its next attack.