Chemical weapons or ensnaring attacks would be most useful
Chemical attacks don't care about the size of the target, just its chemical composition. The assassin beetle is a particularly nasty example of chemical weapons on this scale.
In addition to chemical weapons that burn or otherwise disable an opponent, ensnaring your opponent in a sticky secretion would also be effective. Cockroaches use mucus to immobilize attackers.
Also, ants are well known to just rip enemies apart with their giant mandibles. Never discount a giant set of pincers.
Weapons involving the swift application of mass won't work as well at smaller scales
Little things hitting each other is fun to watch when it's small puppets on a stage but on the scale of fractions of a millimeter, it's just not effective. Given the force equation:
$$F=ma$$
as mass goes down, acceleration must go up to compensate. (For the comparison, let's use momentum which avoids figuring out how long it takes for the pollen grain projectile to slow down.)
If a $12~\text{g}$ arrow traveling at $121~\frac{\text{m}}{\text{s}}$ is sufficient to kill a human at $30~\text{m}$, let's scale down to see what happens with our tiny humanoids, while ignoring silly things like general relativity, material strength and fluid viscosity at small scales.
Let's say the tiny humans want to use arrows scaled exactly the same as normal sized humans. The ratio of $50000~\text{g}$ human to $12~\text{g}$ arrow is $4167:1$. Assume that the tiny humanoids weigh $15~\text{mg}$. Their tiny arrow projectiles would weigh: $0.0036~\text{mg} = 3.6~\mu\text{g}$. For size comparison, these tiny humans are shooting tiny grains of pollen at each other.
The momentum of a $0.012~\text{kg} \cdot 121~\frac{\text{m}}{\text{s}} = 1.452~\text{kg} \cdot \frac{\text{m}}{\text{s}}$
To achieve an equivalent momentum for our pollen grain, it will need to be going:
$\frac{1.452~\text{kg} \cdot \frac{\text{m}}{\text{s}}}{1.23\cdot 10^{-11}~\text{kg}} =118048780488~\frac{\text{m}}{\text{s}}$ or about 393 times faster than light.
Note that $c = 299792458~\frac{\text{m}}{\text{s}}$.