Are you talking about a guide for the campaign or for the battles?
Because I think asking for both might be a little out of the scope of this thread, heh.
Here's some tips from me:
Campaign:
1. Try to conquer something ASAP with your starting army. Preferably within the first three turns or so. It'll give you a massive advantage over everyone else.
2. Unlike something like CK2 or EU, there's no real benefit to being at peace in Total War and there's no real penalty to being permanently at war. Hence the name. Sadly, diplomacy has never been a strong suit of the series. So basically you always want to be conquering things.
3. Having lots of shitty cheap units is usually more useful than having a small army of supermen. One of the reasons why I found Shogun 2's MP got harder as you went was because you had to bring less and less men as you levelled up. So fighting a complete scrub could be very much more difficult than a high level one. Try fighting 4000 spearmen with 800 samurai. It's hard! Same principle applies here. Having more bodies is good, unless those bodies have completely crap morale like peasants. Also, when in doubt, just get spears.
4. It's a very good idea to have ranged units, but try not to overdo it. Only the super late game ranged guys tend to be good enough to win battles by themselves. Most of the time, you'll be relying on your melee to determine fights. (this does not apply to horse archers) As a general rule, try to have most of your army melee and some of it ranged.
5. Don't rush to build and upgrade buildings just because you can. The costs can very easily outweigh the benefits. If you're not sure as to the benefits of getting an upgrade, don't even bother. You can very easily beat a very hard difficulty campaign without upgrading a single building if you wanted; since you will be pouring all the money into your tier 1 army which would steamroll everything and you'd increase your income by raiding and conquering. That's pretty boring though. Basically, watch your food supply and make sure you have enough money.
6. There's usually no real reason to have units guarding every single city. If you have guys sitting around doing nothing, you should probably use them in a real army or disband them to save costs. Be careful though, if you're surrounding by water (say, you're Rome), then the AI is prone to launching amphibious attacks on you. However, most big cities can defend themselves pretty easily due to the AI being pretty stupid when it comes to siege battles.
7. Agents can basically win the game by themselves. Also they'll prevent the AI from cheatspawning units due to them having vision over the enemy you're fighting.
Battle:
1. This might sound controversial but due to the times and technology of the era the game is in, there really isn't much room for tactical brilliance. Prior to the advent of gunpowder, basically every major battle outside a siege boiled down to this:
- Skirmishers and archers harass the armies from the front, before moving around as the armies close.
- Infantry lines clash into each other.
- People try to flank each other (usually done with horses).
Basically do that. Try to have more men.
The exceptions are usually just cheese strats along with the masters of cheesing, the horse archers.
2. The most important part of your army is your infantry line. In Rome 2 though, most line infantry are pretty good with morale so they don't just run away willy nilly. Their purpose is to get into a big fight in the middle and prevent most of the enemy army from moving so your cavalry and other dudes can go around the rear and wreck shit. This applies in singleplayer and multiplayer. Multiplayer Rome 2, Shogun 2 (not FOTS) and Medieval 2 all revolve around who can flank better. Romans and other guys with super strong infantry have the benefit of being able to hold off really huge hordes with a relatively small amount of men. Again, none of this applies if you're cheesing with say.... horse archers. Then you just laugh as the enemy cries.
3. Don't underestimate ranged infantry, but don't rely on them too much either. Unless you're horse archerman, ranged units can't win by themselves as eventually the enemy will reach you and you will die. Even if you killed 80% of him, it wouldn't really matter because you lost the battle. Just 10 Roman legionaries can very easily route 180 archers. However, when you're defending a siege, you'd probably want a majority of your army being ranged. Just because of walls. This also applied to Shogun 2, Fall of the Samurai. It's very fun to charge guys relying on only guns. Sure, you lose almost all your guys but the enemy is boned when you reach them.
4. The AI is actually pretty fearsome in field battles, especially on higher difficulties and when the unit counts are high. However, it has a number of flaws you can exploit:
- If the AI is getting reinforcements one at a time (because there's too many dudes on the map), it will charge them at you one at a time like a moron.
- AI in siege battles for both defense and attacking is worthless.
- The AI has no idea how to unload troops from boats properly.
- The AI will mindlessly chase horse archers around until it routs from exhaustion.
5. Ambushes can backfire on you really badly. For example, if you ambush an army consisting of mostly heavy infantry, your ambush won't do shit since they'd be all clumped together anyway. It's a bit strange, but trust me on this.
Thankfully, I think the battles, strategy, and potential for multiplayer will be more fun with Warhammer rules. Because there are guns, flying units, magic, cannons, flamethrowers, and zany stuff to alter the dynamics. After all, who cares about unit lines when you can drop a meteor on that clump of dudes?