This trick is not an exploit, it's simply a more efficient way of doing things.
If you're like me and are impatient about getting the next rank for your general, then this tip is for you.
As you may or may not know, a general gains experience from a battle when he enters the battle commanding an individual stack. Therefore, instead of grouping all your generals together into one stack and attacking the enemy, move your individual generals off the main stack just before attacking the enemy. Make sure that the generals are close enough so that they arrive as reinforcements during the battle. Their mobility should ensure that they will arrive at the battle line quickly.
In Shogun 2, up to 4 of your stacks can participate in a battle (1 attacking stack plus 3 reinforcement stacks). That means that after the battle, you can get up to 15 XPs for a Heroic Victory for each of 4 generals, which is a lot better than if you grouped everyone into one stack (in that case, only 1 general can get up to 15 XPs for a Heroic Victory).
In addition, I prefer to leave the attacking stack without a general and just have 3 generals in the reinforcing stacks. That way, there is a chance that a new general will be promoted from the ranks. It's a good idea to always leave 1000 koku in the bank for this eventuality since that is how much it would cost to recruit a new general.
Soon, all your generals should have so many ranks that no one can figure who's in charge anymore.
This works in Empire Total War as well though the difference is that there's no limits on the number of reinforcement stacks. Presumably, this should work for all the Total War games starting from Rome, though I haven't tried it on them myself.
Have fun.