For starters, a zweihander was actually more of a thrusting weapon than a cutting weapon, used more like a spear than a sword for the most part, unless the wielder was out of formation in a looting situation. If you look at the martial art manuals for zweihander use, you'll see that their wielders - at least those that survived more than one or two battles - fenced with them rather than winding up to deliver king hits. Their advantage over a regular sword was their length and leverage, with their users hands a couple of feet apart (one grasping the hilt, the other grasping the blunt part of the blade behind the thrust-stop spikes), they could push through a single-handed weapon parry, not with momentum, but with leverage.
So, I can't imagine a professional landsknecht swinging his zweihander around for any purpose other than to frighten non-soldiers.
Secondly, being a largely thrusting weapon, a zweihander is likely to be none too sharp, as the OP has stated. Sharpness is effectively a pressure multiplier. If the wielder of an edged weapon applies a force, F to the hilt of his weapon over a surface area, A1, and applies that force via the edge of that weapon to a target, then the force F is applied over a smaller area, A2. Since pressure is force divided by area, pressure, P at the edge of the weapon is (naively):
P = (F x A2)/(A1^2)
Obviously, minimising A2, by sharpening, allows a higher pressure to be applied. Zweihanders achieve this not so much by having a sharp edge, but by having a sharp point, which is easier to make small than a long edge.
Of course, since weapons are swung or thrust from some point distant from their intended target, the force applied to the target via the edge/point is dependent upon the force the wielder applies to the weapon over the time that the weapon is swung prior to and during impact. The blow then becomes a question of impact energy.
So, if the OP's homunculus is to swing a zweihander (against all common sense, unless it is immune or higly resistant to potential counterattacks) and cleave a fully armoured soldier in two, then a number of factors come into play:
Unless the zweihander has been sharpened to a degree more akin to the sharpness of a scalpel or a Japanese samurai's katana, this will mean that a higher amount of input force will be required.
Oiling or greasing the blade - or potentially even plating it with lead - will assist by reducing the friction between the blade and the cut edges of the opponent's armour.
Plate armour came in several grades, from thin and even rusty iron or mild steel, to thick high-carbon steel proofed against bullets. The latter obviously being far more expensive than the former, with many variations in between. A 'fully plate armoured opponent' may have been wearing thin mild steel that a blow from a merely human opponent could be expected to penetrate if delivered with strength and skill by a sharp pointed weapon, or they may have been wearing thick, high-carbon steel siege plate, against which almost any blow could be expected to be more likely to knock the wearer down than to penetrate. We will assume cheap, thin mild steel plate armour that has suffered from rust during its career, and is now effectively thinner than originally manufactured, even if it is now polished.
The capabilities of the wielder of the zweihander now come into play. Being big and muscular helps in applying force to the hilt of the zweihander, but being big and strong is not all there is to it. Muscles can only contract so fast, and the closer they approach their maximum contraction speed, tbe less force they can apply. Animals achieve high strength by greater leverage - a higher joint-to-muscle-length to limb-length ratio, and achieve speed by making this ratio smaller. Both can be achieved by having big muscles and a small joint-to-muscle-length to limb-length ratio. If magically enhanced, the big muscles may contract faster and more forcefully than usual too.
The technique with which the blow is delivered also plays a part. By "winding up" for the blow, bringing the zweihander around behind the wielder's body and then delivering the blow by applying muscular force not only from the arms, but also the waist and hips, the greatest force can be applied to the hilt of the weapon over the greatest time, resulting in a higher velocity.
The grip the wielder of the zweihander has on the ground also plays a part. Apply more force to the blade than the grip between the wielder's feet and the ground can take, and the wielder's feet will slip, reducing the overall force over time applied to the blade.
Just where the victim of the blow is hit will affect the blow's effectiveness. Obviously, a hit to the abdomen, with the blade passing through one of the intervertebral disks rather than bone, and the blade also hitting the armour at a joint so that the metal need be less cut than merely bent out of the way would make the end result easier to achieve. This would be best achieved with the attacker's shoulders level with the victim's waist given the typical design of plate armour waist articulation.
Obviously, the victim failing to block or parry the blow is very important. Perhaps they are engaged with multiple foes, or have dropped their weapons, or are simply caught off-guard.
So, to ask "how much stronger" obviously depends on all these factors, and can't be answered naively. However, assuming that all else is optimal, the wielder would not need to be greatly stronger than a very strong human, and as other factors become less optimal, the strength required increases to the point where it is impractically high.
To attempt to give a naive figure, if all else is optimal, perhaps twice as strong as a dedicated bodybuilder would be the right figure, but the creation would also be faster and heavier, and may even have clawed feet.
Whether or not the zweihander survives unbroken during this event would be dependent upon the negative factors I have already mentioned, as well as the quality of its metallurgy, construction and its sharpness being points in its favor.
I base this on historical records of merely mortal samurai cutting unarmoured prisoners in two at pretty much every place and angle imaginable, the most difficult successful cut being recorded on the blade as proof of its quality. Famously, one prisoner, being informed that he was to be executed by a cut at waist level, rather than exhibiting fear, quipped that he wished that he had been given more notice, so that he could have swallowed rocks that would have chipped the samurai's precious blade.
Additionally, Japanese records of no-dachi wielding samurai in battle record said samurai cutting horses in two through the body on occasion, the no-dachi being an oversized curved sword like a katana that were in actuality used in the manner that it is only imagined that zweihanders were used.
