So a welding machine.
These come in two basic variants chemical and electrical. Both require the storage of lots and lots of energy somewhere in the suit. The more energy efficient way to deal damage is bullets, but that is what everybody is already doing. In history there have been some experiments with flamethrower suits as a military weapon, but they were impractical and more dangerous to the wearer that to the enemy.
With that out of the way on to our awe inspiring villain. Unless this devise is only used sporadically as a final trove the energy storage requirements soon get out of hand. Watch the development of the propane torch lightsaber by Hacksmith on YT. They had to hide a propane and oxygen tank out of screen to fuel the thing for long enough to be impressive. But suits for hero's and anti hero's are big energy hogs anyway, so lets assume the technology exist to have lots of portable energy.
So lets assume a sufficient source of electricity. At least tens but more likely hundreds of kilowatts of energy. Like that of a battery pack of a very fast electric car. Then a plasma cutter like device would be an option. The plasma would be generated by electrodes and then heated to impossible temperatures with the help of alternating magnetic fields. Or maybe a microwave. If all the energy would be supplied by the electrodes these would erode to quickly.
Because of the low density the temperatures must be much higher to inflict large amounts of damage in a reasonable time. The benefit is that the plasma can be blown into a direction by a compressor fan. This can also reduce the wear on the suit, since the hottest part of the plasma is at a small distance from the device. Assuming energy is no problem, this is the best solution, since it doesn't consume fuel.
915 MHz microwave plasma downstream source running at 60 kW