Spear skills have two major applications. In the middle row, they support the front line and build meter against the enemy's front and middle row. In the front row, they make use of their type advantages to rain down destruction against the enemy's back line units. Spear skills don't have any real specialty, but instead have a variety of effects that can fit many situations.
Notable skills include:
Kapu Kuʻialua is surprisingly fast for being such a low cost, high damage, and high crit skill.
Impale and Wide Sweep are both cheap AoEs, but have restricted range that requires the user to be on the front line for best results.
Salmon Leap is one of the only skills in the game that let you attack from a full 5 rows away. Very handy for hitting dangerous back row units from any position.
Great Spear: Mighty Warrior is unique in that it has very high damage and will never weakens with troop loss, though its limited range requires specific placement.
Draculea and Groza are both very strong AoEs that apply handy status effects to any survivors.
Be wary of using Batsuzan Gaisei unless you know you'll be able to wipe the board, as any survivors will have their delay notably reduced.