How do I get past the monster on the first screen?

First of all, you shouldn't try to use the mask to scare it.

But scaring the monster with something a little more real will help.

Have you tried seeing what's in that coffin on the left edge of the screen?

Magician will be able to open it for you.  The only catch is that he has
to cast on the "S" shape above the coffin, and not on the coffin itself.