Probably shouldn't say this, but I'm a big fan of poisons and acids, and neither is capable of actually melting the body. Acid itself is actually very bad at it, even highly concentrated forms. The most toxic acids actually kill by normal chemical poisoning, not melting. If you want to melt a body, you're looking at something like concentrated NaOH, which is actually alkaline. I don't know if you're aware of how acids and bases (non-soluble alkalis) are defined, so I'll explain. Ignore this if you know. [spoili].To be an acid, you have to be a proton donor. To be a base, you have to be a proton acceptor. The term "proton" is interchangeable with "Hydrogen ion". Acids, which always contain Hydrogen (HCl, HF, H2SO4), are acidic because they donate a Hydrogen ion when in the presence of a proton acceptor. The more hydrogen ions you donate per mole of acid, the more acidic you are. [/spoili] Now for the relevant stuff: Hydroxides are alkaline because they give off hydroxide ions, OH-, which react with an acid's H+ to form water. High concentrations of OH- ions such in concentrated NaOH can actually pull H+ ions off non-acidic molecules. They're kinda assholes like that. Biological molecules contain a shit-ton of Hydrogen. When you remove a Hydrogen ion from a biological molecule, the molecule becomes unstable and splits into two smaller biological molecules. Even DNA can break down this way. Eventually, you get to the point where all the solid biological molecules have been broken down into smaller chains, and the smaller the chain, the lower the melting point. When you get small enough, chains have a melting point below room temperature, and the natural state of the person being dissolved becomes liquid.
You could also melt them with temperature, but that'd be harder because of this thing called Oxygen in the atmosphere. If you were to put the human in a vacuum and somehow remove all oxygen from their body, they'd already be dead. But, then if you heated them up to about 200 degrees, all the water would be driven off and you'd get a slightly smouldering puddle of molten human.
Anyway, short lesson on how to melt people there. Take what you will from it.