Six tells you that you will probably find the means to clear the gate in the Mushroom Forest.
Thortists shoot on sight only if you enter their camp from the tunnel, which is pretty logical because it looks like you are trying to sneak up on them.
That said, I have similiar feelings. Deep Caverns are devoid of anything that made the whole game great if you ask me, constant backtracking and respawning enemies are just tedious and not fun, not to mention the game has practically only one ending since none of your choices matter in the end, just a few different lines of dialogue.
Great game overall, but Deep Caverns don't hold up to that.