Brigades can be any combination of units between 2 and 4.
Some like combined arms units; I tend to find that they get in each other's way (ie making initiative charges difficult since the archers need an order to move out of the way first).
I like to organize brigades by role.
For example, in my example list-
Shooty brigade
2 archers
2 bolt throwers
Shooting has difficultly doing much, so combining into one formation makes it easier to concentrate your fire and maybe do something. Good brigade to field in the center, or to deny a flank. Few opponents will relish going head first into this brigade.
eagles
As others mentions, by themselves. Most efficient on a flank, using homeback and orders to charge flanks in ways that the enemy can't anticipate. Not a line unit, and will be attritted fast... but annoying and effective. Tough learning curve.
A few options; one is two brigades of two silver helms and use the reavers independantly; another is to add them to one unit of silver helms. Put these on the flanks. Just because you have a 4+ AS doesn't mean you need to charge frontally. Use your leadership 10 to get flanks
reavers
4x silver helms
7 spearelves
Either two brigades (3 and 4 units) or three brigades (2-2-3). Use these to back stop the shooty line and extend your battle line. Gives you a center line of ~3 brigades with one in reserve, and two units of flankers.
Use your general in the first turns to move your shooters to a good position, and get the infantry into a solid battleline. Maintaining your battleline without giving up flanks is important in the begininng. Getting out of position early will lose you the game.
Use the wizards to get your flankers moving up. As the game develops, move your general where the decisive point will be. Worry more about what you are going to do to the enemy then reacting to the enemy. Battles are won by forcing your opponent to get out of position responding to you.
When you get to a critical fight, throw in the wizard on a dragon. Never on a close fight- only when you know you can win, and there is little opertunity for the enemy to counterstrike. Once committed, its a priority target for the enemy.
the buff spell is excellent.