Map 30 playthrough help

I have reached what may be the final large room where there is a skull face on a wall, shooting out little cylinders that then spawn monsters (first time this thing appears in the game). There are two switches present - one on the bottom-right island and then one on the central platform - I have used both, then got at the platform just below the skull wall, but don't know what to do from there.

Firing rockets at the skull itself doesnt seem to make a difference. Noclip shows that there is an impaled head of RMS behind the wall (lol) but I cant tell anything else.

Thanks for any help.

This may help.

Cheers :) Surprised they're so similar (not that I remember the normal level, was many years ago when I played the PlayStation ports).

Too hard for me, but have just proven how to do it in god mode.