August is probably the toughest mission in the game.
The only way that I managed to beat it was by being a bit cheap in the way that I took out Dirk. Also, taking him out ASAP is a must.
My strategy as follows:
[spoiler:eycrumu0]He always starts off on the one map with the grass beside the base. I captured the base, and sent a unit there to guard it, then hid a Melodist in the grass right beside it. On the next turn, he runs over to completely murder that unit, and just stands there afterwards without capturing the base. On the next turn, I sent a Fencer there, and had the Melodist boost their stats. After that, probably 3 hits or so, he'll go down. After that, it's all about making sure not to let anybody capture your two bases, while focusing on carefully on clearing out one other map at a time.[/spoiler:eycrumu0]