Build 1: Arc Lightning (adimixed fire) genasi Academiy Mage Infernal Prince fire-blaster +19 attack base, +.75 (from academy, +.75 from enervation v. reflex = 83% accuracy. 1d6+26 brutal 1 = 25.37, +3 if both targets hit = 54.82 fire and lightning DPR total to 2 targets. 3.28 round kill, which is acceptable, but not particularly good. Flame Dragontooth wand is better than an accurate staff of ruin this level, and there are no particularly compelling crit effects to go for incendiary daggers. Significant MAD.
Build 2: Arc Lightning (admixed fire) Tiefling Infernal prince Windrider with mark of storms. +21 attack base, 1d6+26 brutal 1 = 26.27 dpr, 56.84 DPR with slide 1.
Build 3 Winged Horde (admixed thunder) Human Primordial Adept Windwalker. +19+2 attack v. will, 1d4+21 = 21.77 dpr, 75.10 total DPR, at-will. slide 1.
Notes for Build 1
A 4e wizard pyromancer has many options. I don't misdoubt that you're having trouble building an "impressive" nuker.
At the bottom of this answer will be all the builds used to evaluate the suggestions up top. Step 1 is to build a genasi fire-blaster. The guide recommends genasi for non-fireblasters, so it's time to run the numbers for your at-will.
Stats are... remarkably tricky. You need int primary, str primary (Taking a page from the sorcerer handbook when you're adding your secondary stat, there are some strong arguments for even primary-secondary.)
But it gets worse: You need dex 13 (Dual implement spellcaster), Wis 13 (enlarge spell), and con 13 (pyromancy zone)
Now, presumably you're not going for genasi lightningblaster through mark of storm (which has its own MAD, but wind-rider is an excellent PP). Most of the PPs aim for the level 16 feature, so finding an appropriate level 11 thingy is an interesting challenge.
The default recommended PP for me is "Academy Master" simply because of its level 11 benefit that provides a (approximate .8 to hit at optimized levels of accuracy.) The choices change based on element, though. The master of flame PP only really "competes" (and then, not well) at level 16, though it does offer some fascinating synergies with winged horde. Theme of Infernal Prince for accuracy.
For build 1: Arc Lightning genasi fire-blaster +19 attack base, +.75 (from academy, +.75 from enervation v. reflex = 83% accuracy. 1d6+26 brutal 1 = 25.37, +3 if both targets hit = 54.82 DPR to the encounter as a whole. 3.28 round kill, which is acceptable, but not particularly good.
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level 13
Genasi, Mage
Level 1 Apprentice Mage: Pyromancy Apprentice
Level 4 Apprentice Mage: Evocation Apprentice
Level 5 Expert Mage: Pyromancy Expert
Level 8 Expert Mage: Evocation Expert
Master Mage: Evocation Master
Arcane Admixture Damage Type: Arcane Admixture Fire
Elemental Manifestation: Stormsoul
Arcane Admixture Power: Arc Lightning
FINAL ABILITY SCORES
Str 20, Con 11, Dex 14, Int 22, Wis 11, Cha 9.
STARTING ABILITY SCORES
Str 15, Con 10, Dex 13, Int 17, Wis 10, Cha 8.
AC: 22 Fort: 22 Reflex: 22 Will: 18
HP: 69 Surges: 6 Surge Value: 17
TRAINED SKILLS
Arcana +17
UNTRAINED SKILLS
Acrobatics +8, Bluff +9, Diplomacy +7, Dungeoneering +6, Endurance +10, Heal +6, History +12, Insight +6, Intimidate +9, Nature +8, Perception +6, Religion +12, Stealth +8, Streetwise +5, Thievery +8, Athletics +11
FEATS
Level 1: Elemental Empowerment
Level 2: Superior Implement Training (Dragontooth wand)
Level 4: Dual Implement Spellcaster
Level 6: Destructive Wizardry
Level 8: Wand Expertise
Level 10: White Lotus Enervation
Level 11: Arcane Admixture
Level 12: Fiery Blood
POWERS
Level 1 Mage At-Will Powers: Arc Lightning
Level 2 Mage Utility Powers: Mystical Debris
ITEMS
Flame Dragontooth wand +3, Accurate orb of Nimble Thoughts +3
====== ======
Notes for Build 2:
Tieflings are all about fire buffing. Wind-rider gets us +1 to attack and +con to damage for lighting which synergizes with the +con zone bonus from pyromancy.
====== Created Using Wizards of the Coast D&D Character Builder ======
level 13
Tiefling, Mage, Lyrandar Wind-Rider
Level 1 Apprentice Mage: Pyromancy Apprentice
Level 4 Apprentice Mage: Evocation Apprentice
Level 5 Expert Mage: Evocation Expert
Level 8 Expert Mage: Pyromancy Expert
Master Mage: Pyromancy Master
Arcane Admixture Damage Type: Arcane Admixture Fire
Arcane Admixture Power: Arc Lightning
FINAL ABILITY SCORES
Str 9, Con 18, Dex 14, Int 22, Wis 11, Cha 13.
STARTING ABILITY SCORES
Str 8, Con 15, Dex 13, Int 17, Wis 10, Cha 10.
AC: 22 Fort: 20 Reflex: 22 Will: 19
HP: 76 Surges: 10 Surge Value: 19
TRAINED SKILLS
Arcana +17
UNTRAINED SKILLS
Acrobatics +8, Bluff +13, Diplomacy +9, Dungeoneering +6, Endurance +12, Heal +6, History +12, Insight +6, Intimidate +11, Nature +6, Perception +6, Religion +12, Stealth +10, Streetwise +7, Thievery +8, Athletics +5
FEATS
Level 1: Mark of Storm
Level 2: Hellfire Blood
Level 4: Hellfire Arcanist
Level 6: Dual Implement Spellcaster
Level 8: Wand Expertise
Level 10: Superior Implement Training (Dragontooth wand)
Level 11: Arcane Admixture
Level 12: Secrets of Belial
POWERS
Level 1 Mage At-Will Powers: Arc Lightning
ITEMS
Flame Dragontooth wand +3, Accurate orb of Nimble Thoughts +3
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Notes for build 3
Winged horde is way too fiddly to go genasi, since we can't afford a huge str investement. The trick here is that we can spam a area burst 1+1+1 (3) party friendly zone of thundering psychic damage. (also, by going with the battleweaver PP, you can slow. All. enemies. With this. Your GM will throw dice at you.)
Going from my calculated AoE targeting maths, a area burst 3 targets approximately 3.45 targets throughout a battle. Combined with perma-CA through so many sources and an improved crit range from a bloody feat... gets us an effective +21 v. will // 1d4+19 psychic and thunder damage. (1d6 if you want to shrink the zone). 20.07 DPR per target... 69.24 total damage.
====== Created Using Wizards of the Coast D&D Character Builder ======
level 13
Human, Mage, Lyrandar Wind-Rider
Level 1 Apprentice Mage: Evocation Apprentice
Level 4 Apprentice Mage: Illusion Apprentice
Level 5 Expert Mage: Illusion Expert
Level 8 Expert Mage: Evocation Expert
Master Mage: Illusion Master
Arcane Admixture Damage Type: Arcane Admixture Thunder
Human Power Selection: Heroic Effort
Arcane Admixture Power: Winged Horde
FINAL ABILITY SCORES
Str 9, Con 16, Dex 14, Int 22, Wis 14, Cha 12.
STARTING ABILITY SCORES
Str 8, Con 13, Dex 13, Int 17, Wis 13, Cha 11.
AC: 22 Fort: 20 Reflex: 23 Will: 21
HP: 74 Surges: 9 Surge Value: 18
TRAINED SKILLS
Arcana +17
UNTRAINED SKILLS
Acrobatics +8, Bluff +9, Diplomacy +7, Dungeoneering +8, Endurance +11, Heal +8, History +12, Insight +8, Intimidate +9, Nature +8, Perception +8, Religion +12, Stealth +10, Streetwise +7, Thievery +8, Athletics +5
FEATS
Human: Echoes of Thunder
Level 1: Mark of Storm
Level 2: Staff Expertise
Level 4: Superior Implement Training (Mindwarp staff)
Level 6: Dual Implement Spellcaster
Level 8: Enlarge Spell
Level 10: Aggressive Advantage (retrained to Resounding Thunder at Level 12)
Level 11: Arcane Admixture
Level 12: Phantasmal Destruction
POWERS
Level 1 Mage At-Will Powers: Winged Horde
ITEMS
Resplendent Gloves (heroic tier), Headband of Intellect (heroic tier), Mindwarp staff of Ruin +3, Crystal orb of Nimble Thoughts +3
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OK, this is a melee heavy party outside of the cleric.
The scout, scoundrel and Paladin will likely always want to be in melee range (or just out in the case of the Scoundrel).
You and the Scout will be trying to charge someone every round. Your responsibility should be to remain in melee for the scoundrel's chosen target so the he has someone to flank with. The scout should do his best to remain out of melee range after his turn.
If Themes are active, I highly recommend the Guardian theme as you're the toughest of the strikers so taking a hit/encounter for the Scout or Scoundrel is a good trade.
I think you've missed the best opportunity here though. Surprising Charge, with a Gouge. (feats: Weapon Prof: Gouge, Weapon Expertise: Two-handed Weapon, Surprising Charge), grab a Vanguard Gouge and you're doing 5d8 + str + dex + 6 on every charge you make with CA (which is super easy to get here). Plus you'll also qualify for Headsman's chop which you can pick up at 6. Assuming 20 str and 18 dex that makes your to hit and damage with CA:
str 1/2 lev exp stance charging CA mag weap prof weapon talent
5 + 2 + 1 + 2 + 1 + 2 + 1 + 2 + 1 = 17
And damage that looks like
weapon sup charge van dex str arms exp slayer magic weapon
2d8[B1] + 2d8[B1] + 1d8 4 + 5 + 2 + 1 + 2 +1
That's 5d8 (four of them brutal) and +15 static damage at L5 for 39.5 per hit. With a 90% hit chance that's 35.7 damage per charge (includes crit damage).
Best Answer
Build for charge, then take charge enhancing items
There are some excellent charge items out there that make an at-will MBA or MBA equivalent better than encounter or daily all encounter long. Most of the DPR King builds rely on that or multi-attacks combined with high static modifiers like from wintertouched and lasting frost.