M2: On the campaign, I'd always keep friendly relationships/alliances/assassins with the Papal States. Yes, they all result in the same thing. The amount of popes I killed is ridiculous.
On the battlefield, on my Milan campaign half my army was usually Genoese crossbowmen/militia. They RIP through other ranged units, charging infantry and cavalry, and they are amazing in melee, to the point that they can take castle walls from spearmen or even heavy infantry. If you arent Milan, and you're at risk of needing to fight them, take them out as soon as possible.
For most of my campaigns on M2, cavalry was the way to go. A nice little trick for cavalry: if you hold ALT when you doubleclick an enemy to charge, your cavalry are more likely to use lances. Lances are vital to a good charge.
Also, try and take the settlements south of Morocco. There's only one path through the desert, so if you stick an army there they are perfectly defended and can focus on finance, which is good since they're surrounded by ivory, gold and slaves. Whoever you are, having those settlements are ALWAYS useful.
I have hundreds of hours on M2, to the point I could spend a good hour or so explaining all my tactics.
For Empire, artillery is very, very important. One barrage of Canister shot on charging infantry will make them rout in all but the most extreme circumstances.
Oh, and Fire By Rank is possible the best infantry technology to get. If you are against an army that has it, and you don't, you will take SO many more causalities.
If you want to get some colonies in India, get there before the Marathans win (I'm pretty sure they will ALWAYS win if left alone). I've always noticed that Bengal, the most north-eastern province of India, is never defended well. It also has a port and such. But be prepared to defend yourself from both Indian factions. Bengal is part of the Mughal Empire, and the Marathans are VERY aggressive when it comes to Indian territory.