Consider all these spells to do friendly fire.
Hydromancer:
Geyser: A massive geyser bursts from the ground, dealing AoE damage and knockback or stunning the units (non-linear Impale)
Dehydrate: Removes water from the target's body, dealing damage and slowing it severely.
Hosedown: A stream of water that bounces from target to target, removing all buffs.
An alternate one, that requires being the caster being in water/ cast on a point that is in the water to work. If not in the water, the Hydromancer must cast the Flood spell in order to cast all the others.
Flood: (Requires a few seconds of channeling, then remains on the battle field for a certain length) The field of battle is suddenly filled with water in a large circle around the caster, reducing movement and attack speed for all units in the water (Modify terrain by lowering it and adding the "water" doodad).
Water Walk: When cast on a unit, that unit is not slowed by the Flood spell. At higher levels, can be cast on large groups of units.
Riptide: Casts a wave of enchanted water in a line towards a point. Enemies caught in the wave are pulled along, taking damage as they are dragged along the ground.
Summon sharks: Summons sharks (duh) with Devour. If the Flood spell ends while the sharks are still alive, they are stunned for a few seconds and die.
Whirlpool: (Channeling) Creates a whirlpool at a target point. All units within range are pulled towards the whirlpool, where they are stunned and continuously take damage (and spin around).
Pyromancer:
Scorched Earth: Sets a large area of the field on fire, dealing damage over time to any units trapped in it (Modify affected terrain to look burnt).
Man on Fire: Sets a target friendly unit on fire, dealing damage over time. Any units within range of the affected unit also catch on fire, dealing damage, which spreads to other units, which deals damage, which spreads... You get the idea.
CLEANSED BY FIRE: Target unit is caught in a pillar of flame that does damage and removes all buffs.
Banked Fires: Creates an invisible ward that occasionally emits an AoE burst of flame, damaging all units caught in the blast.
Burn The Witch: Sets fire to a target spellcaster. The spell damages both the target's life and mana; once mana runs out, damage taken is doubled.
And of course, the classic fire spells: Flame Strike Firebolt Volcano Rain of Fire Incinerate Immolate Lava Spawn...