It's well known that with the correct frequency applied that sound, and sound alone can shatter a wine glass. These demonstrations are always spectacular, so let's build a weapon on the same principle.
Since we are working with sound, above a certain threshold, a sound wave will just pulverize the target. The CDC lists 10psi of overpressure as
Most people are killed
so let's use that as our upper bound for power requirements for this weapon since we want to liquify the person, not instantly pulverize them.
Further, we assume that we can overcome the high natural elasticity of skin to cause it to liquify.
The OP states that this will be human mounted weapon so safety precautions need to be made to ensure the weapon carrier doesn't get liquified too. While ultrasound is still sound, it is "aim-able" in that pointing the ultrasonic transducer sends sound in that direction and not anywhere else. As long as the weapon bearer isn't "looking down the barrel" then they should be okay.
The 10psi overpressure threshold set out in the Assumption section is 700 times the limits of human hearing (130db). Speakers that generate 110db of sound often require thousands of watts. Granted, the energy from normal speakers is spread across a much wider area than an ultrasonic transducer would but I think that sets a decent lower bound on the power requirements for this this weapon. Given that also don't know the power requirements to achieve destructive resonance in human tissues, the power supply may be asked to provide megawatts of power but since we don't know how much power it takes to liquify human flesh, let's just hand-wave and call it good.
Regular speakers just don't have the power output required or the directionality that we want, so let's go with ultrasonic transducers. Let us assume that a megawatt class ultrasonic transducer can be created and matched to adequate heat dissipation tech.
There is no one frequency that the human body resonates at, or perhaps there is but the power requirements to do so amount to getting hit by the blast wave of high explosives.
At a minimum, you only need to know two frequencies: muscle and skin to count as "liquifying the target". Muscle alone will just make the target fallover. Skin alone just make the target look naked. This leaves the horrifying prospect that the brain are still intact but the target's body is liquifying. If the nerve endings are still functional then.....Death shortly follows but looking down to watch your body turning to liquid.....that's nightmare fuel.
Killing someone may not be as effective as just maiming them by liquifying their skin. Wounded soldiers take significantly more resources to treat/transport than a dead soldier.
Time for Effect
I have no numbers but I think the effects would happen very quickly, just a few seconds. This makes it an effective battlefield weapon because you can't expect a target to stay still for a minute or so.
Since an ultrasonic transducer by definition emits sound at greater than 20Khz, we will need a second transducer to use interference to generate the target frequency. Both transducers can be fitted to a gun-like mount linked to the power supply backpack. Heat dissipation from the power source and the transducers will be important considerations. Add a control to allow the weapon carrier to select a target frequency with some presets for skin, bone, etc. However, access to the raw frequency permits the carrier to experiment with finding the frequency for stone or metal.