All you really need to do is mount up after dispatching the trash pack just before the chamber that leads down to Brutallus' area. Then ride as fast as you can for the window - you may get dismounted (I didn't, but I can't swear that it won't) but just keep going, using any movement speed or travel boosts you have (Blink, Heroic Leap) and make sure you get through the broken window before Madrigosa says "Hold, friends" - as long as she hasn't started talking yet, you can drop through the window and into the open area where the two stand.
Once you're down there, they still have their little squabble, and you can't interrupt it - neither is targetable in any way during their fight - but you can get to see it more clearly. I was actually pretty impressed - for something that happens entirely in game, with no cutscenes or special models rendered, just two big critters duking it out it's surprisingly cool to watch. Madrigosa ends up seeming a lot less silly when you're right up there watching her light up the whole space with her weird magic attacks, freeze him in place and set half the place on fire - it seems a lot more reasonable that she thought she could take Brutallus when you can actually see the whole thing.
Of course, in the end it doesn't work out for her - she spent too much time jawing and not enough time on the ridiculously bright magic show. But it was still pretty fun. In the end, Brutallus ends up winning in his usually jerk way. He didn't even drop anything good.