Much of this is more or less opinion. There isnt that much difference between vertical and horizontal archery. The difference in power between bows and crossbow-type weapons depends on the kind of materials used for the propulsion mechanism and the type of mechanism.
There were basically four types of mechanisms used to propell projectiles.
- Muscle Power
- Counterweight
- Mechanical Deformation
- Torsion
Muscle Power
Many earlier ranged weapons used muscle power to propell projectiles over short distances. It began with throwing stones and spears, but quite soon got supported with levers and slings, creating weaponized slings and spear throwers. They were both quite accurate and quite powerful for the time over shorter distances.
This would be a prime candidate for your scenario, although its usefulness will depend on the kind of armor and equipment the enemy is likely to field.
Counterweight
Not feasible.
Couterweight based weapons are good for catapults, but are much too heavy and large to be used as a carried weapon. You would get much more use from all of the other options.
Mechanical Deformation
Yes, essentially bows and crossbows. With these it comes down to preference: The power of both depends mostly on the materials used and the size / form of the limbs. They are being loaded differently, and they use different types of projectiles. Crossbows seem to be easier to aim with, as they can be aimed "rifle-like", which seems to need much less training than using a bow, due to not needing to hold the tension of the limbs manually, and the way a rifle is aimed (especially at closer distances). You can ready a crossbow beforehand without much effort and let it stay ready some time, where with the bow this isn't really possible.
With loading the main difference seems to be that with a bow you dont need to take your eyes off of your target, I believe the loading time is negligable when used with similar limb strengths. However, you could use much stronger limbs if you assist the tensioning of the crossbow limbs with a lever or a winch, both of which isnt possible with a bow. However, loading a crossbow with a lever or winch will take more time than tensioning a bow of lower power (obviously).
I'd say this is more a matter of preference. If the priority lies with readyness, ease of aim or raw power, go with a crossbow. If Situational awareness and loading time is preferred, use a bow.
Torsion Power
This is similar to mechanical deformation power in that its power depends on the material of the torsion material, the length of the limbs and the length of the lever. You will not need as long limbs as with mechanical deformation, as the force will depend on the torsion material, not the limbs. A major example for weapons of this type is the Roman Ballista. You can achieve great power with shorter limbs at the price of loading time.
Use this if you need a stronger weapon, but don't care about loading.
Projectiles
- Stones or Bullets
- Spears
- Arrows and Bolts
- Chemicals
- Bombs
Stones, Rocks and Bullets are good against unprotected targets and, depending on the weight of the rock, against hardened targets the same way you might use a warhammer against armored targets. Its unlikely to penetrate a target with these, the only way to harm multiple target is either use a large enough rock or let the force of impact throw the target against another (if its that superhuman). Rocks are also much more available ;)
Spears are practical and have a lot of mass behind their tip, making them deadly on short distances. However they are most useful when they are able to penetrate the target. They have more impact on non-penetrated targets in comparison to bolts or arrows, but not really that useful. They are well suited for combat though, as can be seen with the power of spear throwers (like the atlatl), Roman Pilums(although not technically a spear) and Greek Peltasts javelins.
EDIT: apparently, both the Romans and the Greek used a sling in a similar way to the atlatl, called Ankule or Amentum, to propel their javelins over longer distances, which also gave them spin.
Bolts and Arrows have good range and penetrate quite well, cut well and can be used to spread fire. While with some kind of arrows, you could penetrate multiple targets, but since you destry the aerodynamics of the projectile by going though a target, you're unlikely to hit anything (or even at the right angle to penetrate another).
Obviously, if the Bolt is large enough, it has similar properties to larger rocks in that it will hit multiple targets (which has been used with Roman Ballistas).
Chemicals are very dangerous for their size. In medieval / ancient times, greek fire and even quicklime could be used, although I think quicklime is only found later in the 17-18 hundreds. Not sure though. Both were highly effective weapons.
Bombs: Well, you know... possibly, though I'd gauge incendiaries like greek fire to be far more likely.
