Have a character on your team get their weapon fully upgraded in single player mode. You can also have no characters on your team miss during an entire level in single player mode. This option is unlocked at the battle mode team types menu.
When you must get the boat downstream, kill off the two Sintrek, the Grenadak, and the other monster. Do not worry if one of your characters dies; just take out the monsters. When this is done, destroy the two stones in the river, then the bridge (completely destroy it), then do the same for the water mill. Then, destroy the dock. Do not be in the boat when you do this. After the dock is destroyed, jump into the boat, release the controller, and an intermission sequence will start.
Don't worry if one of your characters dies; they will come back to life after you finish the episode. However, to make sure you have a nearly 100% chance of winning, get on top of the roofs when no monster is there. Once this is done, you can snipe away at your enemies. After shooting, go all the way back to the mountain that touches the roof, but do not go down. Shield, because the monsters will shoot your building. If you are all the way back, you will not get hurt. Do not worry; the building cannot crash and you will be ok doing this.