How it plays out:
Hiruko jabs at Kakashi with his tail, which Kakashi evades. Hiruko's senbon are not so easily dodged especially when Hiruko's left arm launches too many for Kakashi to count. The attack can't be dodged. Kakashi uses Earth Release: Earth wall style to stop the barrage.
Both pause, Kakashi behind the wall with Hiruko too far away to be attacked. By this time Kakashi and Sasori have figured out Kakashi must be able to 1) get close enough to hit Hiruko and 2) hit hard enough to destory it. Chidori can break through, but Chidori is a stab and would only damage Hiruko, not render it completely useless.
Sasori doesn't like giving Kakashi time to think and smashes the wall with his tail a quarter to his right forcing Kakashi to go to Sasori's strong side. Hiruko shots more senbon at Kakashi who evades them until Kakashi is close enough to strike.
Sasori crushes Kakashi with Hiruko's tale. Poof, its a shodow clone. Kakashi smashes Hiruko with rasengan. While behind the wall, Kakashi summoned the clone and sent him to Sasori's stong side while Kakashi went to Hiruko's weak side. Hiruko didn't see Kakashi move to the weak side because the senbon shoot from Hiruko's mouth forcing his eyes away from his weak side to aim at the shadow clone on Hiruko's strong side.
Kakashi backs up and prepares The Hidden Mist Technique while Sasori summons the Third Kazekage. Earth Wall won't stop the Iron Sand, so Kakashi chooses to hide from it. The mist becomes thick, Sasori and Kakashi vanish. Sasori doesn't use poison gas even though it would cover a wide area because Sasori wouldn't be able to tell if Kakashi was above ground or not (Earth Style: Hiding like a mole). Instead, Sasori has the Third Kazekage use the Iron sand to create a fan, blowing away the mist.
Sasori spots Kakashi quickly. Kakashi is exposed and stares at Sasori wide eyed fearing the next attack from the 3rd Kazekage. The 3rd doesn't hesitate and levels Kakashi with the Giant Spear. However, this time its the Lighting clone. The bolt rushes though the spear to the 3rd! Sasori releases the 3rd.
Since chakra strings can attach to other chakra and the lighting is formed of elemental chakra; it is possible that the bolt would follow the strings from the 3rd to Sasori. The Iron Sand conducts the bolt well and presents a path to Sasori: Clone-> Giant Spear->3rd's mouth-> 3rd's body (with Iron sand inside)-> Sasori.
Kakashi has made it far. It's obvious that Sasori's offense power far out matches Kakashi's. All Kakashi can do is counter and wait for an opening. With 100 Puppets on the way, Kakashi's chakra nearly gone (rasengan, lighting clone, sharingan, Hidden Mist) Kakashi is done. Kakashi doesn't have a known jutsu to stop 100 puppets. Sasori summons the 100 and its over.
Kakashi kills Sasori. Kakashi took the second while Sasori searched for him to use a genjutsu. Sasori thought he summoned the 100 and killed Kakashi but it didn't. Kakashi ends it with 1000 years of Death.. jk.
Let's say Kakashi didn't use the genjutsu, Kakashi still would've gotten the opening with Silent Killing. Sasori is almost all offense, Kakashi is all about countering and waiting for an opening. Its a great match-up for Kakashi.
Wild cards:
Kakashi's 1000 Jutsu. We've only seen 29, 29 of 1000. that's not even 3%. Kakashi has shown the ability to use Lightning, Earth, Water, Fire, and Wind (when he showed Naruto how to cut a leaf and Rasengan is considered wind release). Kakashi has at least 3 of the eight gates and who knows what else.
Does Kakashi get the Executioner's Blade?
Sasori's last scroll. What the heck is it!!!???? It's been killing me to know!