Serini CREATED an entire hill. We know that the rift was at the surface, and that the rocks of the ravine is magic "multidimensional stone" (thanks @Ruck for that callback), so it's likely the region was terraformed to build a hill on top of the Gate.
Also note that the other dungeons can be pretty much anywhere, since their entrance is a teleport spell. Which means that hill just has to contain "Backstage" and the Final Dungeon. (The Final Dungeon couls also be under another artificial hill, far from Kraagor's tomb, sure, since its entrance is also a Teleport spell, but I don't think so : Serini wouldn't want to have her homebase too far from the Gate she's guarding, just in case someone managed to find a way to bypass her deception.)
It also means that the Final Dungeon can't be too deep, since it has to be above the original ground level. It is inside the hill, not under the hill. But yeah, there is probably a healthy chunk of magicrock between the Gate and the surface.
Or we'll just see one panel of X and RC arriving somewhat singed to the Gateroom, with a "Well, that was annoying" comment, and that's all we'll ever see of "Team Evil's extraordinary adventures in the Final Dungeon of Doom".
We know Serini thinks her dungeon will just weaken and delay Xykon, but can't stop him.
If you look close, it looks like V has 2 "holes" in their chest. And the polar ray looked like it hurt when V tanked it.
V still has these holes (along with a LOT of fire damage) on the current page. So maybe V just has that many hitpoints?