I see poor marg never got any help here, and I'm stumped too.
I can only see one option:
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×-I can't touch the door in the northeast due to the hot tiles, so I can't make Halph walk to the pressure plate to open it.
-I can't manipulate any wubbas or goblins into walking onto the pressure plate.
-I can't access the pressure plate myself, due to the bombs and the orthosquare.
So nothing can press it, right? Well, not quite. Halph won't press it if ordered to... but that's not the only time he moves! He also tries to follow Beethro if you bump him.
This leaves the one possibility I can see: get Halph to follow Beethro, and somehow (probably with the help of the wubbas and goblins) convince him to go through the southwest quadrant in order to do it.
So, uh. How do I do that? Also, I'm kind of hoping Halph's movement preferences will magically take him over the plate, but if that's not what happens I have no idea how to do that, either.