Kakuzu's Earth Release: Earth Spear increases his defensive power by making his skin as hard as diamond. Even though he doesn't need to perform hand signs, he doesn't use it all the time giving Sasori opportunities to poison him like how Kakashi took a sample of his blood.
Sasori killed the Third Kazekage, the strongest Kazekage and was able use his Iron Sand technique. Sasori can shape the sand to create a defensive wall like Gaara's Shield of Sand known as his Ultimate Defense, to protect himself from Kakuzu's mask's elemental attacks (ex: Kakuzu' fire and wind release together and his lighting release). Gaara's ultimate defense was able to withstand Deidara's explosives and his specialty, C3. Iron Sand is stronger than the sand Gaara uses because it's recreated by the Third by analysing the One-Tailed's host. It's like an imitation of the Shukaku's abilities, but with iron sand instead of common sand.
Kakashi was able to withstand Kakuzu's Wind Release: Pressure Damage. If Kakashi survived, Sasori should be okay too because by turning his body into a puppet, he increases his defense. Even Sakura's punch wasn't able to break him in pieces, but only seperated his body parts that Sasori reassembled in no time. Sasori's body is not made with wood like others think, but is made with his own flesh, because it's a human puppet. He certainly added other things inside to make it that compact. Kakuzu might think that he is dead without knowing that he had already switched body. Sasori could use this opportunity to poison Kakuzu.
I don't agree with people who say that Kakuzu's Fire Release: Intelligent Hard Work can melt Iron Sand because after his attack was lunched, some trees were not completely destroyed. It's a powerful technique, but Sasori's Satetsu Shigure, Kesshu or Kaihou can go through it.
Satetsu Shigure are iron bullets. They are so fast that it's impossible to see it coming and to dodge it. Kakuzu has no choice, but to block it. His elemental attacks won't work against it because Satetsu Shigure can go through lighting, wind and fire. Kakuzu can block it, but he can't maintain this technique forever. It takes a lot of chakra control. Also, he covers certain area of his body (his hand, his head...), but we don't know if he'll have time to cover his body entirely. Before he puched down the wall of the temple, he hardened his arm, but it was progressive, but he was fast enough to defend against an explosive, Choji...
Satetsu Kesshu can be manipulated to create different weapons depending on the situation. They can be used like Satetsu Shigure to attack, but can also be used for defense.
Satetsu Kaihou can be used as a distraction because it spreads in spikes that branch off irregularly, making it almost impossible to get a clear overview of what is happening. Sasori can use his Memory-Concealing Manipulative Sand Technique on Kakuzu when he is distracted to confuse him or seal away a part of his memory.
Also, Kakuzu can't use any metallic weapons because of the Third's magnetic field.
