Metall Daddy is really hard to fight in OpenGL :(
I thought so too until I beat him last night while only losing about a third of my health. Just stand near one of the corners and when he starts to jump, start strafing away from the corner along the wall. He won't crush you, most of the smaller mets stay away, and you can hit him with drill bombs fairly easily.
I beat him ages ago in Software, like, the second day the game was out. I just switched to OpenGL, tried my hand at a boss rush and it's a lot harder to judge depth...
Weird I get more perspective issues when I'm in software mode
Software is Doom's render. It doesn't have true freelook. It just scrolls up and down
Here is a screenshot of exactly how it works.
(http://img183.imageshack.us/img183/5364/freelookmyass.png)
This is why sprites don't get distorted in a way...