Where Is Wreckers Cave Divinity 2?

Wrecker's Cave is a sub-area of Reaper's Coast in Divinity Original Sin: 2.

How do I get into Mordus excavation site?

Alternatively, use the entrance near the Wrecker's Cave - Cove waypoint to enter the area. Or sneak through the cave to the far north and use teleport to reach Mordus without fighting any other battles. These shortcuts will lead to an immediate fight with Mordus without being able to clear the dungeon normally.

Should I kill Zanisima?

Can be killed without penalty for 3875 XP. You can still interact with her for relevant conversations and gain extra options through the use of spirit vision after her death. Beast and Ifan both have unique dialogue options wither her.