Here's my advice/ opinions:
Orbs and Balls: Because of their splash damage, they are great when used with lock-on off. They are very easy to dodge for even remotely experienced PvPers, so use them and target them at the end of your opponent's roll. Know the range of your fireballs; it isn't very much, and it's very easy to roll back out of. When casting with lock-on off, remember to "aim with both sticks"--pretty much: left stick is horizontal aim, right stick is vertical (you want your character to face where you're aiming, and your camera to look there). For best effect, you can aim your body at the last instant before throwing the ball, that way you can catch your opponent even more off guard.
Combustion: great to thwart would-be bs-spammers. Also good to use when an opponent is getting back up (I would only say to use this if you're ill-equipped to take on your foe, otherwise it's a pretty low tactic).
Whips: Decent range and multi-hit, but be wary of opponents who have high poise; it isn't very strong, so they'll walk right through it and attack you while you're caught in the animation. Mostly, I see this spell as useful for distracting your opponent, baiting them, then using the bait to spring some trap on them. What the trap is, is up to you! (Another spell, a parry, a bs, etc.)
Tempest/ Storm: already well explained above, but I want to add a warning: be careful when using them against opponents who have ranged attacks--Mages especially. Those spells have a rather long cooldown after you've cast them, during which you can't move. Against melee opponents who will dodge by backing away, it isn't a problem. But if you're against any enemy who uses range, they can get a few free hits on you during this time--CSS is especially devastating!
Mists: I have practically no experience using these. I toyed with the idea of using them for visual cover while I cast some other spells, but haven't gotten around to testing the idea yet...
Edit: How could I forget Iron Flesh and Power Within?! Iron Flesh I've never been a huge fan of, but power within was love at first cast for me! The latter is great in almost any situation, except maybe when you get summoned by an inexperienced player who won't think to flask you while you're wailing on the boss for them and your HP is getting low. Be sure to cast it with an un-upgraded pyro glove, though. It can also be used with the Lingering Dragoncrest Ring to slow the rate at which it drains your HP. Iron Flesh used to be alright for some bosses, but it's been nerfed... And it might once have been borderline-usable for PvP (only against fools who would try and hit you head on) but now everyone knows to either backstab you to oblivion, or just taunt you from just outside your melee range because you move like a turtle!
Final note: while casting any of these spells, always be wary of turning your back to your opponent. You don't want to get backstabbed from a mile away while you're casting Fire Tempest!
Last edited by BLA1NE on Mon Apr 09, 2012 1:11 am; edited 1 time in total