Technique

demonic ice mirrors

Haku's most powerful, and advanced technique is the Makyou Hyou Shou, also known as the Demonic Ice Mirrors. This technique creates multiple ice mirrors out of water which surround all sides and above the opponent. Haku then enters through one of the ice mirrors and his image is reflected through out all the mirrors, giving him much advantage over his opponent. The opponent will then have a hard time locating the real Haku and they must also have to be on guard as to where the next shower of needles will be coming from. Haku is able to travel from one mirror to another in the blink of an eye, and his opponent's movements will seem to be in slow motion to him.
The mirrors which are produced are made from frozen water, though technically ice is meant to melt under heat, these mirrors need a higher, intense heating-power to melt then normal. They are also quite strong and is near impossible to shatter.

weapon

Haku's weapon of choice are needles. Although needles aren't very deadly and don't have a high risk of inflicting much damage on the opponent, Haku is highly skilled with them and is able to instantly kill a person. Needles can be a deadly tool if used properly, having dead accuracy in throwing, he is able to hit someone's vital points on the body or render them into suspended animation (seemingly like death).
These needles are called "senbon" and are oftenly used by Hunter-Nins, who have thorough knowledge of the human body structure. These needles are also those used by doctors in acupuncture therapy.