For me I usually have my Princepes in the middle line, with them autofiring their javelens. This usually takes out a good number of enemies. On my flanks, I usually put Triarii/Pedites Extraordinarii, and usually one or two of these units in a second line in the middle to support where the enemy charges the hardest.

With that said, most of the enemy army is engaged with the middle line and the flank forces, I concentrate my one or 2 cavalry units on one flank, and continually crash the rear of the enemy on that one flank, between the flanking infantry and cavalry, they route, and I continually do this down the line until their entire army is surrounded or routing, where I send my entire cavalry force to whipe them up.

I usually dont use Guard mode as I like the counter charge my men sometimes make after routing the enemy, it gets them chasing when I would more then likely order it anyway.