Firstly, as mentioned already, the deathsquito is a plains mob. It has only 10 hp and (i believe) has a weakness to fire, or at least, from what I've witnessed, they explode when they get anywhere near a fire or bonfire, so I imagine you could kill them with a single hit from a torch, but almost ANY weapon will do.... and this is IMPORTANT.... Never try to run away from deathsquitos... They are too fast and giving them a free shot on your back is a sure-fire way to a nice gravestone (or collection of gravestones).

Deathsquitos have a predictable attack pattern, once agro'd they will fly in a straight line directly toward the player, and pause briefly before taking a shot on the player.

Shoot them with a bow as they fly straight at you, or prepare to block / melee and hit them when they reach you.

Pick up needles once it's dead.