Reproduced on EGA Floppy Version with ScummVM and with DosBox.

You can use the following glitch to significantly speed up Level 5 (one with a big face, the mushroom and a scary skeleton).

The glitch is: when the skeleton appears from the coffin, you can hold SPACE to avoid the damage from it. This would constantly switch between gobliiins and I guess the switch takes precedence over the damage trigger.

You can also spam-click pickup action on an empty spot with Oups to skip the damage, but I am not sure if it's faster, and the other gobliiins do not seem to have a similar method.

Did not find any application on other levels so far.