True, but since DMC Mundus is still sealed in the actual statue and can't get out anymore, that still gives DmC Mundus an upper hand because he can use the literal world of Limbo as a weapon.
You do know the conditions of the fight were that if DMC Mundus were freed and fought DmC Mundus head on like he did Dante and not still stuck in a statue.
Then there is the issue with that since DMC Mundus is sealed in the Underworld the only way DmC Mundus can fight him is if he head into the Underworld and fought him and he would lose the advantage of manipulating Limbo since he is in hell and hell is not Limbo.
Also, Mundus can unseal himself it just takes time and also head into the Human World.
Plus due to DMC Mundus ability to create a warped reality and trap people in it wouldn't he be able to do so in Limbo and trap DmC Mundus in it cutting off connection to the real world or Limbo and maybe even the Hell Gate.
So I still don't get the hell gate immortality thing. Due to the hell gate he is immortal due to the fact the hell gate consumes human souls and uses them as a power source so the power of the hell gate is really human souls and nothing connected to hell itself....well sort of. Plus wouldn't Mundus need to be near to the Hell Gate to draw power from it and if not and he is somehow connected to it via a spiritual link.
Plus even with Limbo as a liability what makes you think with Limbo he can defeat DMC Mundus if he couldn't kill DmC Dante with Limbo? What is he going to do open up a huge hole in the city to drop Mundus in it? I mean Lilith could've manipulated Limbo and she still got her a$$ kicked by Dante no sweat.
Plus fights must also incorporate tactics into it. All DMC Mundus would need to know is that DmC Mundus is immortal unless the hell gate is destroyed and all he needs to create a demon pawn like Trish and trap DmC Mundus in the warped reality and have the demon he created destroy the hell gate. DMC Mundus isn't as stupid or impulsive as DmC Mundus considering that DMC Mundus have used a fair share of dirty tricks (Trish, that trap Nightmare set, and making him fight his own brother) and tactics to deceive Dante and kill him while DmC Mundus main downfall was caused by his own incompetence such as allowing the woman or demon who is carrying his child or heir he cares so much about walk around despite the fact a Nephilim is going around f*cking with his plans and getting duped by Dante leaving his precious Hell Gate unguarded and worst of all letting them trade Kat a person who was in his building and saw a lot making giving her away a rather huge flaw that really ultimately ended him, I would've killed Kat and just bang another demon chick to get another child (Mundus never cared about Lilith but the child and not even the child he needed a heir he didn't need a specific child). So yeah DmC Mundus isn't the brightest guy out there. DMC Mundus main flaw was the fact he ****ed off Dante too much well with Trish and made Vergil or Nelo Angelo keep the amulet allowing Dante to awaken the Sword of Sparda but I guess Mundus didn't know much about the amulet. Personally Nelo Angelo would've been more effective if his identity wasn't hidden and Dante knew from the start he was fighting his brother although it could mean that Dante could talk sense into him if seeing the amulet is enough to give him a headache and leave the fight.
DMC Mundus powers come from the existence of hell or darkness (since he was made from it) so unless you can wipe away hell or evil he will never die.
DMC Mundus situation is easy. Trap DmC Mundus in the warped reality disconnecting him from Limbo and maybe the Hell Gate and if that isn't enough send a demon (since he can create demons out of thin air like nothing) to his building and destroy the Hell Gate.