MANAGING YOUR ARMY
More than most things in AoW2, it's difficult to give general advice about your army in AoW2. The map plays a big factor. Is there a lot of water, mountains, or lava? Flyers will be valuable. Are there a lot of roads? Ground troops will be important, as they can can travel even faster than flyers on roads. Lots of forests? Concealed units become more powerful, as well as units with forestry. Lots of underground? Units with cave-crawling and night vision become more valuable. You get the idea. And, of course, your starting race and the available races on the map plays a huge role in shaping your army.
UNITS- WHAT TO BUILD
In most games, city-produced units will compose the majority of your army. Units cost gold; not only to produce them, but also through a per turn upkeep charge. The upkeep for units is level dependent. 3/6/9/12 gold for level 1/2/3/4 units. This upkeep can really hurt you in mid-game, especially in a map with scarce resources. So although you want to have a huge army, you also don't want to build hordes of units that won't be of much use except draining your coffers. Notice that, from an upkeep perspective, it is more efficient to have a smaller army comprised of level 3 and 4 units than a horde of level 1 units because most level 3 and 4 units are far more effective than the equivalent amount (in upkeep cost) of level 1 units.
Generally, level 2 and 3 units will comprise the core of your army. While level 1 archers are important for city garrisons, and level 2 cavalry and specialty units have important roles to play, you will normally have the best of chance of winning if you crank out large amounts of level 3 units as fast as possible. Most level 3 units can be produced fairly quickly, especially with hurry production, and simply outclass level 1 and 2 units. However, don't just blindly race to level 3s- you need to get some level 1 and 2 units out first so you can gather resources and conquer key points in the early game. But you do all this with the idea that you will eventually start cranking out level 3 units. There are some exceptions to this (e.g. tigrans, discussed below), but this is a good rule of thumb to follow. Another thing: don't assume that one level 3 unit is going to clear out a horde of low level units- that won't happen because of the mob tactics discussed above. But groups of level 3 units become very strong and will defeat larger groups of level 1 and 2 units in the open.
What about level 4 units? In most multiplayer games, I don't bother building them. Yes, they are some of the coolest units in the game and have a cool dramatic effect when you bust them out for everyone to see. The problem is that they have too many required structures and are too expensive to justify their general use. You will lose significant, often fatal, turn advantage when you go for these units. I have played many games where I stuck with level 3 units while my opponent went for level 4 units. By the time they were able to produce the level 4 units, my level 3 units were dominating the map and I had enough of them to take out the level 4 units on the board. There are certain situations where you might want to build level 4 units. If your capital starts with a lot of advanced structures, it becomes more appealing. Also, certain races have level 4s with unique effects that could change the game for a player who is unprepared to deal with them. Even in these situations, though, you could still do fine by sticking to level 3s. In most cases, refuse the temptation to go for level 4s. Trust me in that it will usually work out for you if you play aggressive and make the most of your turn advantage.
I'll give race specific advice in a later section.
ORGANIZING AND DEPLOYING YOUR ARMY
Before we discuss race specific unit advice, there are some general points to using units effectively. You should always have some units scouting at the edge of your borders. Ideally, these scouts will be fast units that are expendable, although any unit will do in a pinch. You need these scouts not only for information, but also to grab free mines and resources before your opponent gets them. Remember, you're trying to build turn advantage and every little bit helps. Scouts need not be in groups- because they are expendable, send off by themselves in all directions. Units with concealment make the best scouts. If you have concealed scouts, be sure to keep them in the forest and in out of the way forests where no one will bump into them by accident. If your scouts get killed, don't fret about them- just send some more off. You want to reduce the fog of war as much as possible so you don't get surprised.
For the core of your army, try to group units in packs of 3 or more (the further along the game, the bigger groups you need). Individual units, even strong ones, are very easily picked off. Packs of units, on the other hand, are quite tough to defeat. It's tempting to race out units as your city makes them, racing them off to their destination. But if you do this you will never be able to stack your units. This is especially important when you start building level 3 units which are quite valuable. Group units with similar movement together. This allows you to make the most of your units mobility. This is not an absolute rule, however, and if you know you will need to combine slower and faster units together (e.g., warlords with wyverns) then do so.
If you have flying units, be sure to build them. They will prove invaluable in city sieges when they fly right over the walls. You can be a little less careful with fliers because you can land them on mountains, ocean, and lava without fear of being attacked by ground units. If you have flying scouts or advance troops, try to land them on hexes that can't be reached by ground troops. And of course, it's a good idea to stack your fliers together if they are going to fight instead of scout.
If you don't have flying units, you need to build some kind of anti-flying force unless you know your opponents (through agreement) are not going to use fliers at all. You might be lucky enough to build units which have web or entangle. Cold strike and lightning strike can also bring down fliers. Certain magic spheres have ways to counter-act fliers (chain lightning and great hail can bring them down, although not so reliably. Static shield and summon spider are useful also against fliers.) But, otherwise, you'll need range units to fight fliers. If you know you're going to be encountering strong enemy fliers, and have no fliers of your own, then you really need to try to keep range units near your strong ground troops. Otherwise, because of the limited retaliation effect discussed earlier, groups of fliers will eat your ground melee troops for lunch. If you know your enemy is going to use mostly fliers, keep in mind your ground troops will have a movement advantage on roads (fliers can't take advantage of roads).
Remember that note about diversifying your forces? When you organize a group of units into a strike force, try to ensure that the group can deal with various situations. Most importantly, it needs to be able to deal with range units and with fliers. If you have units like catmasters or charioteers, then you're problem is largely solved- you mostly can get away with just cranking out stacks of them without worrying about other stuff. But if you have units like runemasters or warlords, you probably want to bring units like crossbowmen or abominations with them.
Priests are useful units to have in a strike force, but they’re also quite slow. They make excellent city defenders, with their healing and magical ranged attacks. I don’t like having them in my main strike forces, despite their healing benefits, because they really slow down stacks.
Also, don't forget about putting a garrison your key cities. Depending on how much wealth you have, you might put at least a minimal garrison in all your cities, to prevent concealed scouts from taking them for free. Range units should be the unit of choice for garrisons. Siege machines are good also, although I rarely use them. Remember, though, that the city needs to have a wall in order to make use of range units advantages in city combat, so be sure to build a wall first if there isn't one! If you have entangling or web units, you might consider putting a couple of them in a city garrison. They can immoblize enemy units that crash the gates while your archers turn them into pincushions. Also, if you have a powerful melee unit like a runemaster, consider having a few of those in your garrisons also. They can block the gates while your archers get more free shots. One cheap tactic to keep in mind is that a unit that is on the gate before the gate is broken must be killed before the gate can be attacked.
ATTACKING WITH YOUR ARMY
A lot of AoW2 beginners are far too passive. They want to sit back and build a huge force before venturing into enemy land. Maintaining an aggressive strategy towards your enemies is vital to winning competitive games. I don't mean you have to rush at them blindly- which can be dangerous, but never let them sit back and build up at their leisure. That is a sure way to set yourself up for loss. Even sending scouts into enemy territory is helpful. The key is to give your opponents the perception that they're being threatened, even if they're not. Many players will not respond well to this pressure and make mistakes. So don't be afraid to attack. Think of it this way: your units are simply draining gold for you if you are not using them.
Now that you've organized your army into groups of 3 or more, hopefully diverse enough to handle a variety of situations, you need to send these units out to attack. Ideally, you are sending out some scouts ahead of your main forces. This will give you valuable information about what you are facing, and allow you to retreat or consolidate forces if you know you're going to be outmatched. Of course, it also tips your opponents that a force may be coming their way. Thus, concealed scouts are the most effective.
Try to move your key forces near the end of your turn in simultaneous turns. This gives opponents less chance to run away, as well as allow you to retreat if you suddenly see something you don't like. It also allows you to move quickly in the beginning of the next turn. Of course, good players will also follow this practice, so there can be a lot of cat and mouse at the end of a turn.
It takes practice to get the sense for what fights your armies can handle and what they can't. Keep in mind that AoW2 has a big luck factor, so you will lose fights that you should have won, and win fights that you should have lost.
Pay attention to domain lines, since spells make a lot of difference in combat. You will obviously want a hero in your main strike force, since they carry domain with them.
Keep in mind that if a hero dies in TC, the domain goes away with him, but if the hero retreats, the domain is there for the rest of the battle.
Remember that in sea combat, units in boats without flying, floating, swimming, or water-walking will not be able to participate in the battle.