You're forgetting many a key thing. For one, it's not that Yuki doesn't feel pain, and in fact damage done to her body slows her down and she can't heal right away. Also, that was Skeith's 1st Form with only Haseo in 2nd form after his First Job Extension. Haseo has two further power ups and several more Epitaphs devoured. There is Skeith the 2nd, an upgraded form after Haseo obtains his 3rd form. Then there's the Lost Weapon, of which only one exists for each of the Eight Phases, and the Lost Weapons also grew and transformed over time (happens in .hack//G.U. vol3.) Not only this, but Haseo can also perform Data Drain outside of Avatar form.
Haseo's Xth form is a result of a near total fusion with his Epitaph, which makes him the most powerful. Haseo defeated Tri-Edge Ovan when in 3rd form both times, even when Corbenik the Rebirth also grew into Corbenik the 2nd with an even more powerful AIDA arm. There's another thing; Haseo had to battle and defeat each of the other Epitaphs in order to gain their powers, and most of the time they were infected with AIDA and were getting boosted. The 1st time Haseo battled Kuhn, the Epitaph user of Magus, he lost control of his Avatar and it went berserk. Skeith is brutal and merciless without control.