And now for some pretty amazing damage...
Human female (lighter weight so she can ride the dire bat)
Str 14+
Wis 14+
Other stats don't matter so much.
1 Druid 1 Mounted Combat, Ride-by Attack - select heavy horse animal companion
2 Druid 2
3 Druid 3 Spirited Charge
4 Druid 4 - select dire bat animal companion
5 Fighter 1 Power Attack
6 Fighter 2 Improved Bull Rush, Shock Trooper
7 Druid 5
8 Druid 6 - dire bat advances by 2 HD; choose 1 cross-class rank in Tumble and the Flyby Attack feat
Skills: Concentration, Ride, Handle Animal
Get a masterwork wooden lance and Druid's Vestments (DMG) -- see if your DM will allow you to buy them at the cheaper price listed in the Minor Magic Item table rather than the 10,000 gp listed in the item description. Or just get a Lesser Metamagic Rod of Quicken -- if you can get this, you don't really need any other magic items.
Cast Spikes on your wooden lance for +2 attack, 19-20/x3 crit range, +6 damage (overlaps with +1 enhancement bonus on the wooden lance).
Attack bonus: +6 BAB, +2 lance with Spikes, +2 Str, +2 when charging
Damage bonus: +6 Spikes, +3 Str, +12 two-handed Power Attack/Shock Trooper/Heedless Charge
Mounted lance charge: +12 attack, damage 3d8+63; crit 19-20 for 5d8+105.
At level 9, select the Lion's Pounce feat so you get two lance attacks on a mounted lance charge. If you can get Riding Boots (DMG2), you can have a free Ride-by Attack feat; this allows you to have Lion's Pounce by level 6 if you swap your druid and fighter levels around a bit. Extra Wildshape would be a nice feat, too, as would Quicken Spell.