Depends on the faction your playing and the faction you are fighting, as well as the terrain.
The most common way to beat an oposing force of a lordparty is to use flanking even if they outnumber you.
A common battle situation for me as playing viking faction against any lordparty roughly 150 vs 300:
Infantry: 50% ~ 60-80 men heavy infantry, consisting of as many svear warriors as you can get, veteran rekr and other heavy infantry units you gathered.
Archers/Skirmishers: 50% Warrior Archers
Shocktroops: ~20 men including your companions in a seperate group. You may want to add additional shocktroopers like elite vikings, ship captains, bandit leaders and alike, in general I use christian troops for this job as they will have a high probability to die
Not that I hardly ever use spearman, they are inferior in many ways, useful in defense but unless you want to micromanage even more using different stack sets, stick to heavies.
The formation looks like this at the start of the battle:
Archers on the left, heavy in the centre shocktroops on the right flank, note that the infantry is deployed ahead to make sure it draws the attention.
All units are in hold fire mode.
HHHHHHHHHHHHH
AAAAAAAAAAAA SSS
When the enemy formation is approaching ~ 100m or less the archers are ordered to open fire, while the infantry slowly falls back
^^^^^^^^^^^^^
^^^^^^^^^^^^^
^^^^^^^^^^^^^
^^^^^^^^^^^^^
AAAAAAAAAAA HHHHHHHHHHHHH SSS
And further....
AAAAAAAAAA ^^^^^^^^^^^^
HHHHHHHHHHHHHH SSS
When the enemy closes in, all units open fire, javelins 20m or less. The archers are now firing into the unproteced flank and rear, the shocktroops engage, the enemy formation either rotates towards them completely exposes them or they suffer heavy casualities on both flanks, The infantry is allowed to engage, depending on the damage dealt and the total strength of the enemy but it does not charge until the odds are clearly in your favor. This usually happens when the enemy lost a considerable number of men, when the enemy falls back to regroup with its reinforcements the infantry charges them, while the archers hold fire to preserve arrows.
Using this tactic I beat enemy stacks twice as large or bigger with less than 6 casualities inflicting 300+
Archers are pretty gimped in this mod, javelins are much more effective but require precise timing and a source for this type of troops. So far I havent tested it on large scale but imagine having 50 skilled javelin throwers on the left flank throwing at close range into the ranks, it will be like a machine gun. Highwayman are particularly deadly in this role. And they are not shabby in mele as well.