In comments, I suggested that we should use force instead of energy as a measure of punch strength. I'm not entirely sure it is the most useful measure, but its what boxers use when we measure the strength of their punches. It also yields really nifty results, which is always a bonus in worldbuilding exercises.
You made the assumption that they can keep their body together when using this super strength to punch. However, nothing says the world around them can keep together, so we're going to have to model the forces holding together objects like people and safes. There are molecular forces which try to hold together an object. If you strike an object, the object "responds," typically at the speed of sound for that medium, distributing the force across the bonds. Ideally, it would distribute the force evenly across the object, but in a real world, sometimes that doesn't work. If superman punches a safe with enough force, he can force the object to deform enough that it cannot distribute quickly enough. In that case, some of the molecular bonds fail, and we see the end result of superman punching a hole right through the safe.
So we can model everything in the world besides our superhero as a bunch of small bits (molecules or larger) rigged together with springs which can rip apart if put under too much strain. It doesn't perfectly model physics, but it does describe the world well enough to give us hilarious slow motion videos.
So the question is what can they do without bracing. Because our superhero is being modeled as "tough" enough to not rip apart, we can treat them as one lump body, with a mass and a corresponding center of mass. If the superhero only uses his superhero strength in a way which does not move his center of mass (meaning his fist goes one way, and his body moves slightly the other way to keep his CM still), he can actually accelerate to any speed he pleases (and speed at impact * mass of flying object = force). So super strength actually does help!
However, if you punch at ungodly speeds (speed of sound, relativistic speeds, etc.) the world around you may not withstand the impact. If you try to maximize your force, by maximizing speed, you don't necessarily maximize damage. If you punch Lex Luthor in the gut, you really don't want the damages to be limited to a fist-sized hole. I'm sure he'd find a way to work around that impediment. You really want to do more damage than that. Somehow you want to hit in a way that hurts his entire body!
This is harder because we've hit the limits of what you can do with just the invincible parts of your body. To do more damage, we need to start using all the resources available to us, like the soft squishy body of flesh right in front of our invincible fists. This is going to sound a bit strange, but we're going to hit softer to hit harder!
Visuals time! We need something on a human scale to capture what we're trying to do. Let's say our opponent is something soft, like a nice moist cake standing upright (maybe the cake is in the shape of a punching dummy). We want to use one of our fingers to strike the cake and do as much damage as we can. Sure, we can just ram our finger through it as hard as we can, but that only makes a 1cm wide hole in the cake. However, what if we had something soft and squishy, but bigger to help us out. Soft... squishy... tomato! For our visual, let's pretend we have a tomato in front of our finger, in front of the cake. I have no idea how this got so strange so quickly, but the visual works out. If we unleash full force, we squish a hole through the tomato, into the cake, but we can only make a finger-sized hole. However, if we can unleash softer, we can give the tomato to accelerate into the cake. Now we can make a tomato-sized hole! All we have to do is hold back just enough to avoid breaking the tomato's skin with our strike.
Okay, normal-human scale imagery aside, our superhero needs to punch with just enough force to accelerate the region they hit to use those molecular bonds to accelerate the nearby region. Then this region needs to accelerate just fast enough to accelerate a bigger region, and so forth. By hitting softer, we hit harder!
This means the effect of super strength is also dependent on your ability to strike wisely, not just hard. If you can strike in a direction which takes advantage of the strong directions of a bone, you can affect a much larger region. You're literally using their strength against them.
To strike harder than that, we need to work with our opponent. Yes, first we hit softer, then we work with our opponent. A lot of things in the body, like non-innervated muscle tissue, is really bad at transmitting forces for us. We'd do a lot better if we could convince our opponent to tense their muscles at just the right time. This is not easy. The easiest way to do this is to make them flinch before you punch them. Then they tense up, and the muscle transmits your punch better.
There is a school of thought which suggests that the most powerful punches are not the big ones, but the small ones. The smaller the punch, the more you can feel what your opponent is doing, and adapt to them. If they tense in one way, you adjust yourself to take advantage of that particular way they tensed up. This could hypothetically lead to punches which begin to use the opponent's mind against them. If you can figure out how they're thinking about the punch, and adapt what you are doing to encourage them to give up a mental high ground in return for a physical high ground, you can strike them right in their core, destroying their will to fight, rather than just their ability to.
So super strength is useful because you can generate as much force as you want. However, while a force of four tons may crush bodies, a force of four ounces, applied properly, may crush a soul.
