In fantasy novels, the epic duel of mages is always a dramatic and flashy affair. In Dungeons and Dragons, not so much. A primary archetype of the fantasy mage duel is that the opponents unleash epic forces to counter and nullify the enemy's castings. But under the third edition rules of D&D, it's not that easy to counter an enemy spell, and a mage hoping to nullify her enemy's magic has an uphill fight.
A would-be counterspeller must jump several hurdles. First, ready an action. Second, identify the enemy's spell with a Spellcraft check. Third, if you have the right spell prepared or have it available, cast the counterspell. Ugh.
The biggest problem? What if you don't have the right spell prepared, or if the correct spell is not part of your repertoire?
Well, the Improved Counterspell feat helps a lot:
That makes it easier, since it broadens the range of spells you can use as counters. With this feat, you can counter a Fireball (level 3 evocation) with an Ice Storm (level 4 evocation). Or with any level 4 or higher evocation.
Improved Counterspell [General]
Benefit: When counterspelling, you may use a spell of the same school that is one or more spell levels higher than the target spell.
Normal: Without this feat, you may counter a spell only with the same spell or with a spell specifically designated as countering the target spell.
In some ways, that makes the sorcerer the best counterspeller, since he can cast spontaneously that higher level alternative spell. But a sorcerer knows a limited number of spells, and may not know a higher level conjuration, say. In that case, maybe the ultimate in countering would be the caster who can Heighten a lower level spell on the fly and use it counter an enemy casting. Ideally, you could Heighten a Magic Missile to level 4 and use it to counter a Fireball (using a 4th level spell slot to do so).
But, alas, if you try that, you trip over the first of the hurdles I mentioned. Look back up there at the first hurdle: Ready an action. The rules say that "You can ready a standard action, a move action, or a free action." But spontaneously casting a Heightened spell is almost always a full round action, as the rules say: "If the spells normal casting time is 1 standard action, casting a metamagic version is a full-round action for a sorcerer or bard." So you cannot ready a spontaneously cast metamagic spell because the casting time is too long.
But have I got a loophole for you! OK, casting a Heightened spell with a casting time of one standard action lengthens the casting time to a full round action. That's why you simply cannot cast a Quickened spontaneous spell. It would be paradoxically self-defeating -- the rule that a spontaneous metamagic spell takes longer to cast trumps a metamagic feat that shortens casting time.
Still with me? Okay, now, to cast a spell with a longer base casting time, add another full round action. For example, a druid wanting to Empower a spontaneously cast Summon Nature's Ally III (to be able to summon as many porpoises as possible, say) takes one round and one full round action to do so. However, what if you use a metamagic feat to cast a spell with a base casting time of less than one standard action?
There's only one of those in the core rules: Feather Fall. Casting it is a free action (or an "immediate action" if you use the rules revision that came later). So casting a Heightened Feather Fall should increase the casting time -- I would rule, to a standard action. That means you CAN cast a Heightened Feather Fall as part of a readied Counterspell!
So if you have the Heighten feat and the Improved Counterspell feat (and you have a decent Spellcraft check and you know Feather Fall), you can spontaneously counter any transmutation spell that your enemy casts!
But what about the many spells that are not transmutations? Clearly, what you sorcerers need are more spells with the shortest of casting times, spells from a range of schools that can be Heightened for use in countering. For example, if there were an evocation with a free action casting time, then it too could be Heightened and used to counter enemy evocations. So go out there and research some spells with free action casting times!
What? You're still here? You want me to do ALL the work for you? Fine. Here, then, are a few spells of my own creation, ready to be used as is or heightened at need.
Necromancy Level: Sor/Wiz 1 Components: V, S Casting Time: 1 immediate action Range: Personal Target: You Duration: See text
Description You grow a ghostly, glowing tentacle shaped from your life force that you can use as an appendage to deliver a melee touch spell. The tentacle can deliver any single touch range spell you cast after the tentacle is created. The attack must be made before the end of the next round. The tentacle is treated as a melee weapon with 15-foot reach, though you dont threaten the area into which you can make an attack. In addition, unlike most other weapons with reach, you can use it against foes anywhere within your reach (including adjacent foes). Attacking with the tentacle does not provoke an attack of opportunity.
Conjuration (Creation) Level: Sor/Wiz, Bard 1 Components: V, S, M Casting Time: 1 immediate action Range: Medium (100 ft. + 10 ft./level) Area: one 5 ft. x 5 ft. square Duration: 1 round/level Saving Throw: Reflex negates Spell Resistance: No
Description You fill a 5 ft. x 5 ft. square with puddles of glowing, glittery, golden paint. Any creature passing through or standing in the square must make a reflex save or step in the paint, subsequently tracking it around and leaving glowing footprints for the duration. Track rolls against those affected are made at +20. Spot checks to detect and to pinpoint invisible creatures who step in Glitterpaint are also made at +20. Material Component: Ground mica in an oil suspension.
Evocation Level: Sor/Wiz 1 Components: V, S Casting Time: 1 immediate action Range: Touch Target: Creature or Object touched Duration: Instantaneous Saving Throw: None Spell Resistance: Yes
Description A flame shoots out of your finger as a melee touch attack. The flame inflicts 1 hp fire energy damage per level, to a max of 5 hp damage at level 5. Also, the flame will ignite a flammable object or structure, but it wont ignite a creature it strikes.
AND, here are a few more neat ones I found with the help of Dr. Google.
A few extra thoughts for optimizing your counterspells.
Of course, you want to keep Dispel Magic and Greater Dispel Magic handy to use for counterspells when nothing else will do. In fact, if your DM is nice, keep your Dispel Magic wand handy and use that for counterspelling -- just make sure the wand has a caster level of 10, if you can swing that.
And when you hit the higher levels, be sure to take the Archmage prestige class. The Mastery of Counterspelling high arcana power -- reflecting countered spells back onto a caster -- is super sweet.