This thread is a little irrelevant until the next update, when everything changes considerably. Currently, tactics amount to choose whether or not to trutle, which weapons you use, and training. Other than that it's just get people into position and let them kill. Considering only marksdwarves get told to move to a position and don't just rush the enemy, there's not even a lot to getting people into position. It'll get a lot more diverse next update with new siege mechanics and the option to get aggressive yourself.
That being said, theoretical tactics for meeting an enemy in the open field? Have a few principles:
-You'll want to soften up the enemy with ranged weapons. Concentrate your melee squads around a core of marksdwarves.
-Keep your forces consolidated! If a group of soldiers or war animals is isolated from the main formation, it can be surrounded and mobbed. Keeping your forces together will mean each unit works at a higher efficiency in terms of inflicting casualties, and has a smaller chance of becoming a casualty. The exception is deliberately sacrificing war animals for an advantage to dwarf soldiers, which we'll discuss later.
-Mix & Match! While training bladed or blunt weapons is less efficient in a mixed squad, during the battle each unit should have diverse capabilities for the eventuality that you fail to keep your forces together in a single, solid mob. This also goes for soldiers based on experience level; if you want to keep those recruits alive so they can keep landing hits, its better to allow a legendary dodger and armor user to take for them. You'll also need legendaries spread around to deal with enemy legendaries.
-War Animals are your friends; they take very little time to train and are easy to replace. Using your war animals as a buffer between yourself and the enemy means they're less likely to maim one of your dwarves. War Animals can also be used to delay enemy squads from approaching your formation, allowing you to keep a temporary numerical advantage. They're also particularly useful for sicking on enemy archers; if they're distracted by something biting their leg, they can't pincushion your dwarves.
Condensed version:
-Mix weapon skills (except archery) and elite soldiers between squads.
-Keep your squads concentrated in a single area, around a core of archers.
-Use war animals to thicken up your squads for a numerical advantage, or use them agressively to ensure your main body of troops only fights on enemy at a time.
-Try to avoid archer fire; it can really trash an otherwise solid formation alarmingly quickly unless archers get a nerf. Either move to engage those as quickly as possible or distract them with war animals.
One thing to watch out for; no matter where you position your troops, they'll currently charge the enemy as soon as they see them. If there's another enemy, they'll then charge them instead of returning to their position. So formation tactics (as opposed to a huge mob of legendary zerg) will break up if you let too many enemies engage them too fast. You'll want the formation to remain put; if it's on the move, people can get left behind and mobbed.
Oh, and bring plenty of food and water. Alcohol too... but water. IDK if they'll set up any supply mechanics or a battlefield medic system, but those are secondary considerations to defeating the Greenskin horde.