His piercing attacks cause Sparda!Dante immense damage, and thats with his regen and really impressive durability (Note: Normal Dante4's stabs cannot even scratch their own DT hides). This goes to the extent of being about to knock Dante out of this mode.
For another look at it, there's his last battle with Dante, after Sparda!Dante wrecked him and left him for dead to burst his was into the human realm. Mundus was in a near death state, his body literally falling apart and extremely sluggish like that of a massive zombie. Power diminished so much he was reduced to hand stomps and occasional eye beams. He was up against Dante1 with access to DT but minus Sparda, and in the end Dante was the one physically exhausted while Mundus just taunts him, calling him weak.
Full power Mundus will have access to his energy attacks, including a mega-laser, elemental manipulation (Lightning, hellfire, Frosts extreme cold temps), corrupting those weaker than him into his slaves, a powerful out-but-not-in shields that can take a barrage of Spada!Dante's attacks before breaking, and reality warping.
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"Gonna need more chloroform..."
"If you look down on us, judge us, condemn us... then you shall fear us!"
The way I see it, Dante from 1 and 4 are around the same level as Sparda's human form. Their basic DT forms would be like Sparda's if he had one, but a true Devil form like his true form is above that of a human/demon's DT.
If Dante1 had a Majin form, it would easily be on the level of Sparda. For DMC4 they intended for Dante to have his own Majin form where they state this power was to far surpass Sparda.
All quotes about Dante4 come from his unused Majin form.
__________________
"Gonna need more chloroform..."
"If you look down on us, judge us, condemn us... then you shall fear us!"
Think of it this way. Sparda has a human and full devil form. While Dante has a human, devil trigger and a full devil form.
Unless you mean that Dante's human side makes him weaker. In which case that's a recurring plot point when a weaker character gains a large demonic power and wonders how Dante can still triumph. He explains that humanity/heart makes demons far stronger, just like with Sparda.
__________________
"Gonna need more chloroform..."
"If you look down on us, judge us, condemn us... then you shall fear us!"
Far faster at this point given they're both above Alastor's speed. Mundus warped an entire island and a vast pocket dimension on whims, a tiny fraction of Argosax's power warped an entire island and maybe more, and Abigail was able to make a continental to planetary sized portal. Basically feats preformed on whims.
__________________
"Gonna need more chloroform..."
"If you look down on us, judge us, condemn us... then you shall fear us!"