There are several ways to inflict a ton of damage with one blow in Dungeons and Dragons. For example, as I have mentioned before, you can go for the sneak attack, or you can optimize your critical hits. But perhaps the best option of all is the mounted charge.
The key is that the Spirited Charge feat inflicts triple damage with a lance. The tripled base damage is nice (3d8 for a medium character, 3d6 for a small character), but the beautiful part is tripling your bonus damage.
Start with strength. Although you CAN use a lance one-handed, you'll WANT to use it two-handed. Then your strength bonus damage is increased by half again. Due to rounding down, the sweet spots are at str 14 for +3 damage and str 18 for +6. Tripled, that's +9 or +18.
Power Attack is also key. Using a weapon with two hands, you double the bonus damage. So if you take a -2 on attacks, you gain +4, which tripled is a +12. At a higher level, if you take, say, a -10 on attacks, you gain a +20 damage that becomes +60 on a spirited charge!
Note that the biggest penalty you can take with a power attack (and so, the largest bonus to damage) is equal to your BAB. So it pays to stick with a better BAB advancement than a lesser. For example, a druid/sorcerer/paladin would be better served, BAB-wise, by advancing into Eldritch Knight rather than Mystic Theurge.
Try to find other ways to add to your bonus damage. Magic lances with enhancement bonuses are good. Spells and bardic music can boost your damage too. Do not bother with effects that add dice of damage, such as flaming weapons or sneak attacks -- those do not multiply.
And don't forget the possibility of death by massive damage. You may find that your enemies are on the receiving end of that rule!
Maxing Your Mount
But it is all wasted if your mount dies! So put some thought into how to keep your mont from croaking, or how to keep the supply of mounts flowing! Here are a few options.
Of course, just buying a horse or a riding dog gets you off the ground. But the survivability of common mounts is quite low.
[Speaking of dogs and horses, let me make a quick aside here about mount size. The disadvantage of being a medium rider is that your mount is large. A horse is not the ideal mount for a dungeon crawl. If you are small, a medium mount, like a riding dog, is much better. A dog can go just about anywhere a person can. Food for thought.]
The Leadership feat lets you have a special mount as a cohort -- a unicorn, hippogriff, or other neat mount works fine. You could get a friendly druid to Awaken a horse or riding dog for you, and then the mount/cohort could gain levels, in fighter, say, or maybe barbarian. But if your cohort/mount dies, your Reputation suffers.
The mage's familiar is too small to ride. You might be able -- especially if you are a small mage -- to use the Improved Familiar feat to call a familiar you can ride, or maybe a familiar you can enlarge and then ride. In this case, you are probably thinking of multiclassing to gain a few of your needed feats, lance proficiency, and a better BAB. Wizard5/Fighter2/Eldritch Knight is an option here. But if your familiar buys it, you either need a raise or you have to wait a year! AND you lose experience points! Ouch!
The Paladin's mount is a better option -- if the mount dies, it comes back in just 30 days, or when you level up. No XP loss, but you do take a -1 penalty while it is gone. Being able to dismiss it as a free action can save it from death, too, since you can take a free action even when it is not your turn.
But the best option of all, respawning-wise, is the Druid's Animal Companion. You can dismiss it freely, suffer no penalty if it dies, and it only takes a day to call a new one, with no waiting period! [If you are looking for a secondary mounted strategy and you have chosen the druid route, check out this screed.]
If you have a nice DM, you might be able to take the Improved Familiar and/or Leadership feat and apply them to a familar or paladin's mount or animal companion. In this way you might be able to boost the survivability of your mount. A multiclassed PC with more than one animal friend may be able to give up one in order to boost the other, maybe by stacking levels. For example, a DM rules that a Paladin5/Wizard5/Eldritch Knight can give up his familiar, stacking Wizard and Paladin levels to determine the powers of the special mount.
Advice From Afar
Of course, I am not the first person to explore this strategy. Here are a few other Web pages with food for thought.
Here's a Mounted Devil that the evil DMs at Wizards of the Coast presented online as a tough foe.
This WotC message board conversation focuses on how to max out a mounted paladin charger.
This pair of halfling NPC foes, offered by OotS creator Rich Burlew, make a striking pair. One rides mounted on a dog and uses spirited charges to good effect.
This En-World forum talks about how to make your Paladin's mount live longer.
This D&D 3.0 prestige class lets a paladin extend his mount's powers by giving up spell advancement. It is overpowered but has a few good ideas you might use in making your own.
So if you love the idea of making your enemy into shish kebab, then mount up and charge it!