This is the formation I used to beat the mud giant. It was more effective overall (for me) than any other formation (that doesn't use Maria) in this section (because I don't have Maria):

Empty, Empty, Main
Calvin, Herja, Ahmose
Empty, Empty, Adam
Sub: Shayd

Adam should stay alive till the end. Calvin drains the boss's morale every 2 rounds. Herja allows more skill attacks from Adam and Calvin. Ahmose stays alive long enough to allow Shayd to pop in in front of the boss (and not in front of a monster).

Note: I had preciseness level 10 on SHayd, and Advantage level 11 on Adam. I think those two are important, otherwise Adam cannot deal enough damage on his own..