If the goal is to melt the target, using current technological ability, the only way to produce sufficient heat in a man-portable way would be a fission heating device, taking inspiration from Corium material (the melted lava-like alloy formed in a nuclear reactor meltdown).
The maximum temperature reached by typical nuclear reactor contents in a meltdown is said to be up to 2800°C in the above linked Wikipedia article. Pure Tantalum has melting point of 2996°C. However, the Terminator endoskeleton is an alloy, so it will have lower melting point. Also, a nuclear material mix specifically designed to reach high heat output with self-limiting nuclear chain reaction should easily reach high enough temperatures to melt even pure Tantalum.
We want isotopes which do not boil away at too low temperature, and which have low enough critical mass so that the weapon is man-portable, and which have reasonable half-life so they are not overtly radioactive without a chain reaction. Therefore isotopes of interest, taken from this Wikipedia article, include at least these:
Isotope Half-life, years Critical Mass, kg Boiling point
Uranium-233 159200 15 kg 4131 °C
Neptunium-236 154000 7 kg 3902 °C
Plutonium-238 87 9-10 kg 3232 °C
Plutonium-239 24110 10 "
Plutonium-241 14 12 "
Using these, and ignoring any environmental and safety regulations on testing, it should be trivial to design a special Corium cocktail, which
- Is safely sub-critical in a hollow sphere or cylinder.
- Is super-critical (has exponentially accelerating chain reaction) when collapsed into a solid mass.
- Becomes sub-critical when it starts to boil, so that the chain reaction immediately stops when bubbles form in the hottest part of the mass (the center), and as a result the mass remains in nice, toasty temperature of about 3200°C...3500°C
- Can have a chain reaction with mass under, say, 20 kg.
Construction of the warhead should be so, that the Corium cocktail is in a form of two hemispheres, or possibly even several separated disks, staying safely sub-critical. When fired from a recoilless rifle, these parts get smashed together into a sphere, which promptly begins a chain reaction and basically melts almost immediately (like, in 100 ms, as designed), but does not explode or evaporate because the chain reaction ends when any material starts to boil, as explained above.
When fired successfully, and melting on the way to the target, the now molten sphere of highly radioactive material at above 3000°C gets splashed onto the target, knocking it down with kinetic energy and starting to melt it. The sheer amount of radiation should also mess with its electronics, at the very least frying its visual sensors.
Now the splash will also stop the chain reaction, because the Corium cocktail is going to become too dispersed. Solution to this is simple. You need several attackers firing at the target when it is knocked down, so it stays down and is thoroughly covered in the Corium cocktail, which will agressively remain near boiling point. The puddle will likely stay molten months, if not years, slowly melting its way through the ground.
As for the delivery system, for example the US Javelin has warhead weight of 9.4 kilograms, while weight of weapon when ready to fire is 22.3 kg. This is a good starting point, and by removing guidance and reducing range and velocity, and increasing total weight a bit it should be possible to reach warhead weight of around 15 kilograms.
The obvious drawback is, this requires a team. Also, due to radiation released, using this weapon is likely a suicide mission, unless you add some remote firing ability. But hey, you gotta do what you gotta do.
If you come up with a way to restrain the target (some kind of net?), then you could tie it down and place a crate of this special Corium cocktail on top of it, and then trigger the reaction by just removing supports keeping parts of the material separate and subcritical. Melting commences, and no one else has to die.