Books & Rules + Models 
Two rules come to mind if I was fighting this army:
P45 Charging Cavalry or Chariots into ‘Soft Infantry’
When cavalry or chariots smash into weak infantry, their sheer weight can often make a real bloody mess of them, leaving very little to fight back.Under the following conditions underneath, some infantry pieces will be classed as ‘Soft Infantry’ versus cavalry or chariot pieces which charge over 7 inches or more into combat.
All units and mobs, which do not carry pikes or spears and have light armour or no armour are classed as soft infantry.
All ‘Flimsy Ranked’ pieces are classed as soft infantry, regardless of how they have been armed. This special rule is fully explained on page 44.
All units and mobs that have been charged in their ‘Flanks’ or ‘Rear’ are classed as soft infantry; this is regardless of how they have been armed. Refer to the previous page to learn more about these vulnerable areas around all pieces. A piece’s ‘Rear’ and ‘Flanks’ are explained on page 44.Where the soft infantry rule applies; cavalry and chariots, both their riders and their steeds gain +1 attack each in combat, whilst the enemy infantry must suffer -1 to hit them back in that fight. After the first round of that combat has passed, this rule no longer applies and all pieces affected by it will return to fighting as normal.
And
P34 Wipe Out Move Bonus
Sometimes in combat the enemy is wiped out before they have a chance to flee, either because all of their models have been killed off, or because a unit piece becomes a non gaming piece or an ‘It’. If this leaves the victor completely free from all combat, then all the pieces freed up immediately gain a ‘Wipe Out Bonus’, which is 6 inches of free movement to spend as they wish. If the freed up piece has a movement value of 12 inches or greater, they gain 12 inches of free movement instead. This free move can be spent on any type of movement its controlling player wishes to use; they may even charge the piece into another enemy if they wish. Charging another enemy after wiping one out is classed as an ‘Unintentional Charge’, which is fully explained on the next page.Though ‘Pursuing’ and ‘March Over’ moves can destroy a piece instantly, pieces which achieve this do not get a wipe out bonus. However, a piece charging into a fleeing enemy piece will get this bonus if they catch and destroy that enemy. The ‘Pursuing’ rules are on page 34, and ‘March Overs’ are described on page 31.
Note: If a piece completely kills off an enemy in the first round of combat after charging and uses its wipe out move to hit another enemy, this is counted as one big continuous charge. Therefore, certain other rules such as the ‘Soft Infantry’ rule and ‘Lances’ might come into play. For instance, a piece that wipes out one enemy and then charged into another whilst they are armed with lances, they will get that weapon’s bonus again. ‘Soft Infantry’ is explain on page 45 and the rules for ‘Lances’ are on page 73.
charging another enemy after wiping one out is classed as an ‘Unintentional Charge’, which is explained on page 35.
Battles can be won or lost on these rules. It all depends if Surbags’ Boys can hold after a Herlanceian Charge. They should be able to see off Heavy Riders, but Grand Riders are point for point the best cav in the game, and in a Wedge formation with a magical command, say +1 to hit… They could have 7 models in the front, each one with 3 attacks each with Soft Infantry , totalling 21 attacks (not to mention the steeds’ 14 attacks too), -3 to the Grunts toughness save when their lances hit and they will Strike First… Yowch…
On the other hand… you do have the Overlord Banner and the numbers to sook up a hariffic charge. So… it could depend if Surbag is in favour with the great Dice Gods on that day.
Also terrain will be another big factor too… And I suppose there is also the question of maneuverability with Surbag’s horde, but lol with your numbers you should swamp the battlefield.
