Soul Arrow: Particularily Great Soul Arrow. Soul Arrow, Much like the Fireball spells in pyromancy, isn't used because there is a better version. Just like you'll never see fireball used over fire orb or great fireball. However, you never see Great Soul Arrow Cast...
The homing on Soul Arrow is magnificient, if an opponent rolls at the same time they normally would to evade Soul Spear, the projectile homes enough that at long range, the soul arrow will still hit them. Doing about 400 damage a hit, it's respectable due to having 20 casts and having nearly no risk to it's caster. Furthermore, it's a great chasing spell. If an opponent is running away, rather then attempt to catch them with a rolling attack, you can cast a soul arrow, and I gurantee it will hit your opponent 100% of the time. Since most people run away with less then 400 HP left, this is good for killing a coward who is trying to abandon the duel.
The heavy versions of soul arrow are just as great in their homing capabilities, and further throw the opponent off with their odd cast time.
Emit Force: This is a faith builds Great Fireball, I swear to it. It does equivelant damage when you use the ring of the suns firstborn and the dusk crown, and it's best used without lockon. Aim it at your feet when someone tries to roll BS you, and you'll ram him for 1K damage on a counter hit. It does the same damage as a great fireball, while deflecting projectiles and having 6, rather then 4 casts. If you're extra evil, you can use the ring of the sun princess to further boost it's damage by 10%.
Gravelord Sword Dances: If you're a gravelord, rank 3, these do 30% more damage. That makes great sword dance do far more damage then fire tempest ever will do, while also getting twice the casts per attunement slot. You can easily one-shot anyone with this skill, as it does 1670 damage on average for me, without getting counter damage. With counter damage it easily hits 2K. The gravelord sword dance is almost as powerful, with just as many casts. Bear in mind if you're not a member of the Gravelord covenant, it suffers a 10% damage loss. So the difference inbetween being a member +3 and not being in the covenant is effectively the difference inbetween 90% and 130% damage.
Far higher requirements, but far higher reward, then fire tempest.
Sunlight Spears: While easily dodgable, if you fight lock-off you can with a lot of skill at aiming, aim at the -exact- spot the person will land their roll. If you do this correctly it WILL hit them, and this often scores 1400 or more damage in a single hit. Be prepared for accusations of lag, however, because not a single opponent will ever believe you led them into that one... perhaps if more people started doing it...... Note, impossible against ninja-roll.
Dark Orb: This skill travels really fast, and it's hitbox is the size of a semi-truck. Great for knocking over shields, inflicting massive amount of damage to backstab fishers, and for catching opponents at the end of their roll. You can use it locked on, the game actually takes care of enough that lock-off fighting doesn't improve results, in my experience.
Karmatic Justice: If you are a pure caster, use it, otherwise, don't. Karmatic justice is sort of bugged, as it does damage based on the weapon currently held in your right hand. That means if you're still holding your talisman, the explosion is lethal, coming in at 1300-1800 damage easily if you've got all magic boosting items.
If you have any other spells, I can probably find a PvP utility for them, tell me and I'll tell you how to turn an underused spell into a major killer. I'd like to make video's of this later when I get my capture card.