It's really pretty easy. Start off the skirmish buying your resource maker (farm/tree/mine/whatever), and with your other builder, get a barrack/range/ whatever. Once the unit building has been completed, you should have 2 farms, or about to get the second. Keep building resource buildings with that builder, while you build unit builders with the other builder.
Here's the good part - On Brutal, the enemies ALWAYS attack your farms. So put a line of them up towards the enemies base, and constantly build your forces. Once they start attacking your farms, you should have enough units to crush them easily. Have your builder make a well/statue/forge, and start upgrading your guys. This is easiest with the Elves, since their Silverthorn arrows obliterate anything and everything. It's hardest with Isengard, because they tend to suck in general.
Once you have a sufficient wall of troops to halt the enemy, save up your points for magical use until you have one of the two level 25 ones. When you are ready, summon your Lvl 15 allies, and use the Lvl 25 skill on the next group of enemies. Now you can rush into their base unhindered, and obliterate them.
All archers works best, they clean the base in no time flat. I find that with the elves, dont bother using Mirkwood archers because they dont get armor upgrade, and take more command points. The only time I bother with the Mirkwood guys is when I have a Castle like Helm's Deep to defend, because their range is unmatched.
All in all, it should take you anywhere from 8 - 15 minutes to crush any Brutal opponent using this method.