Lolcat, can you explain your retinue tactics?
Cthulhu, mercenaries maybe?
There was a wonderful guide written up on the forums with a whole bunch of information. I'll try to find it. I'm terrible at trying to explain the combat mechanics of CKII, they're a little bit all over the place.
EDIT: Couldn't find the guide but I found some good information here.
Unit TypesCombat tacticAs you can see from the combat tactics it's generally best to stack a sole unit type or unit types that complement each other so you get the good tactics. Since you should always stick to cultural retinues (unless it's light infantry or light horses) you generally want to stack those unit types together. Vassal forces will come with a vast mix of units which will influence tactics to be much more random. On top of that they generally have a bunch of light horses
which you must always avoid. LH have pretty useless stats and they will employ tactics that will gimp your other units.
Note that non-cultural retinues will not have any bonuses while cultural retinues will have bonuses equal to the highest cultural building you can construct.
It's always best to find a leader with leadership traits that will benefit your stacked army. If there are cultures with a cultural tactic that will heavily benefit your army, such as Schiltron, you'll want to either employ some from over seas to lead or make a ghetto for renewable leaders.
If you have light horse or light infantry you'll want to employ a non-cultural retinue and mix in vassal forces. Light infantry cultures can survive off self and vassal levies alone since the cultural building will give a massive increase to the brunt of your army.
Do note that to effectively "game" warfare you'll need to be a fairly big size to have decent stacks of retinues. King level and higher is generally required. It also helps to invest all military tech into retinue size as well.