lance
戣 centers on a type of pole weapon: a lance or spear-like arm used for thrusting.
戣 combines 戈 (a halberd or dagger-axe) with 癸, which likely contributes the sound. The character originally referred to a specific type of lance or spear.
The right side 戈 is a weapon, and the left side 癸 looks like a complex pole. Picture a lance with an ornate head: 戣 is that lance.
For キ, imagine a key-shaped lance tip unlocking a shield: key -> キ, and the lance 戣 pierces through.