I'd decided to try making both sides surprised, putting a stream across the roman baseline, and giving the Romans first move to reflect Plutarch's account of the battle which you be found here in his life of Pyrrhus (16 onwards being the relevant sections).
I went with the numbers Plutarch gives as having set out on the trip across to Italy even though some may have been lost in the storm. As regards Pyrrhus'somewhat reluctant Tarentine allies, it seemed best to make their foot generic heavy infantry rather than Macedonian-style phalangites to accord with the passage in Polybius (18.28.10).
(As for Pyrrhus he employed not only Italian arms but Italian forces, placing cohorts of these and cohorts composed of men from the phalanx in alternate order in his battles with the Romans.)
I went with 4000 Tarentine foot, enough for one levy heavy infantry unit, which is a slight reduction from the 6000 that Jeff Jonas proposed in the very useful OOB he did for his Warhammer Ancient Battles scenario, here. If you are reading this Jeff, your scenario was a great jumping-off point, so thank you very much for posting that.
For the Roman forces I went with four legions plus allies to allow Laevinus to outnumber Pyrrhus' men (as Justin's Epitome of the Philippic History of Pompeius Trogas reports he did) without things getting ridiculous.
Here then are the orders of battle.
The 3000 men of the advance guard become 1 veteran phalanx unit and 1 average phalanx unit.
The 20,000 foot he brought over with him become 10 average phalanx units.
4000 Tarentine foot and 1000 Tarentine light horse (both figures basically plucked from the air) become 1 levy heavy infantry unit and 1 average light cavalry unit.
3000 heavy cavalry become 4 units of veterans and 1 average unit. Pyrrhus is attached to his guard as an inspired leader.
2000 archers become one unit of average light infantry.
500 slingers and 20 elephants become one unit of Indian elephants with skirmish support.
The total fighting value of the approximately 30,000 foot, 4000 horse and 20 elephants is 80.
The 27,000 or so heavy foot of the four legions and allies becomes 14 average legionary units.
The 10,000 or so light foot become 2 units of levy light infantry and about 2000 camp guards.
The 5000 or so heavy cavalry become 5 units of the same.
Laevinus is rated as an uninspired commander.
The total fighting value of the approximately 35,000 foot and 5000 horse is 78.
The zones were set at 800 metres across, giving an attack limit of 4. Just to explain the 'surprised' rule in Lost Battles, the main differences between a normal deployment and a surprised one are that surprise limits the number of units that can deploy in a turn to four, with heavy infantry and elephant units counting as two units in the first turn, and the army fighting value for command generation purposes is limited to the units actually on the field at the start of the turn.
So then, to the action!
As you would expect, both sides send forward skirmishers and cavalry on the first move. The Romans concentrate troops in all three central zones, with Laevinus and two units of Roman cavalry in the rightmost. Pyrrhus responds by sending three cavalry units and a light infantry unit forward: the former force to counter Laevinus; the latter to protect the key zone in the centre.
Pyrrhus' light infantry feel a little exposed...
The second turn sees Laevinus roll a 1 for command, giving him a miserable 2 commands plus 1 exemption to work with. He spends 2 to deploy four legionary units in the centre, and uses his exemption to launch a cut-price attack on Pyrrhus' zone, scoring a hit. In return, Pyrrhus double-moves some phalangites and two more units of cavalry to secure his presence in the centre. In the attack from his own zone he shatters one unit of Roman cavalry and leaves the other spent.
Towards the end of turn two Laevinus begins to get a trifle nervy.
Laevinus rolls another 1, giving him 4 commands and an exemption. He brings on another 4 units of legionaries and retires with the spent cavalry behind the safety of their shields. Due to an accounting error he does not have enough commands to bring up support for the velites...
"Why did I not count up my on-table fighting value correctly?" laments Laevinus.
...which proves costly as Pyrrhus now launches attacks in the centre and the centre-right, shattering both units of velites/leves. He declines to advance in the centre, but does advance the cavalry of the centre-right.
In the distance the phalanx begins to mass in the zone behind Pyrrhus.
Laevinus now gets a decent roll for his commands, brings on another two units of legionaries, and sends two units of equites into the far left wing zone, threatening to encircle the cavalry that advanced after defeating the Roman light infantry. Pyrrhus pulls the cavalry back at a cost if three valuable commands, and elects to make an outflanking move of his own on the other wing.
We should have stayed here all along, grumble the cavalry. That way our support might have caught us up.
Laevinus now brings on two more units of legionaries in the centre left and his last unit of cavalry to defend the rear from the marauding Pyrrhus. With a few more commands at his disposal his men are encouraged enough to score four hits on the phalangites on the Pyrrhic left, leaving the entire zone spent and dangerously close to ruin.
Determined attacks by the legionaries throw Pyrrhus' infantry off balance.
An uncomfortable experience for the cavalry as the maniples move into line.
With his infantry line in danger of folding, Pyrrhus' earns again his storied epithet, 'the One'. He decides that he will attack this turn and try to bring on reinforcements on the next. It is a gamble, but his men do experience some success on the left.
The phalangites take a deep breath and pray that reinforcements will arrive soon...
Laevinus urges his men foward to break the phalangites opposite them, but they cannot score any shatters. Elsewhere they have more success as the centre and centre left all score hits. The consul now brings on his last undeployed units (the triarii, naturally!) to support his beleaguered cavalry in their efforts to ward off Pyrrhus' attempt at encirclement. The cavalry score a hit on Pyrrhus' supporting veterans, and suddenly even these fellows are starting to feel the pressure. Their reply though is magnificent: two hits; and knowing that the Thessalians have begun to come up in support, Pyrrhus does not feel the need to exchange cloaks and headgear with anyone quite yet.
It has come to the triarii...
Around this time Pyrrhus is faced with a problem: after rolling another 1 for command he can only bring up troops to support one part of his line. The left or the right? He chooses the right, and relies again on his staunch phalangites to hold on for another turn on the left.
Laevinus is not idle - his men on the right shatter a unit of phalangites, continue to score hits elsewhere, and outflank the Pyrrhic centre-left.
Things are not much better for Pyrrhus' infantry here, either.
Pyrrhus is able to bring up some fresh reinforcements in the left and the centre, including the long-awaited elephants, but it is now the right that is in trouble, despite its earlier reinforcement. There is some encouragement though: all the units in the the Roman centre are now spent, and Laevinus' zone on the Roman left has only one fresh unit remaining.
I think we've seen you fellows somewhere before?
On turn 8 Rome begins by shattering a unit of phalangites and completes the 'spending' of Pyrrhus' centre and centre-right, but without being able to do as much damage as Laevinus would have hoped. Pyrrhus is still unscathed, and the rest of the infantry line totters but holds.
Pyrrhus' line on the verge of collapse.
With the Thessalian cavalry now up in the line, Pyrrhus gives them their head and in a dramatic attack they shatter the stubborn triarii: the rear of Laevinus' lines is now exposed! Elsewhere the attacks are prosecuted with vigour, but the disorder in the ranks prevents a breakthrough.
Those Thessalians are worth their weight...
It is now the Roman's turn to stall. They can manage no shatters, but they do score a hit on the elephant unit, which brings them back into contention on their right. Now though they are surrounded, and it does not look as promising as it once did.
Don't look behind you, gentlemen...
On turn 9 Pyrrhus attacks in the centre, using as many combat bonuses as he can. The breakthrough is made, and with more than four Roman units shattered and the central zone with enemy to front and rear, the morale roll sends the rest of the units in the zone packing. The centre, and with it the Roman key zone, has been won. Now, all they need to do is shatter one more unit... but they cannot manage it.
Moving first on the last turn, the Romans finally cause a shatter and a morale failure on the Pyrrhic right. Only the veteran cavalry stand, but they too are shattered in the next attack, and so a breakthrough by one side in one part of the line is matched by the other in another. Laevinus however is unable to score a shatter on the zone opposing him, so the Pyrrhic army still has a chance to win the day.
The situation at the of the Roman move in the tenth and last turn.
With all depending on breaking Laevinus' zone no combat bonuses are spared. Two hits are scored, and Laevinus, summoning all his powers of inspiration, attempts to rally his men. Two hits... and two rallies! He succeeds, his men remain on the field, and the battle is a draw.
After such a dramatic battle, with so many changes in fortune, a draw seemed to be a good result. Honour was satisfied on both sides.
When it came time to count up the points, I found that Rome had won a close battle on points by 84-68. Rome probably won the game in the last turn when it routed the Pyrrhic left; but Pyrrhus might have won it back had it not been for the heroic performance of Laevinus in rolling two glorious 11s, in defiance of the odds (though obviously in accordance with the will of the gods).
A great game, finished in fine style!