Fire Emblem: The Sacred Stones/Chapter 7: Waterside Renvall

Seize the castle
Send Neimi and anyone else you want north to kill the fighter and mage and use the ballista. You can have her weaken or kill the two soldiers guarding the bridge to make it easier for the rest of the group to get through or have weaker units finish them off. It's recommended that you send Vanessa to rescue Neimi once she uses up the ballista and send her to reunite with the rest of the group.

Send the rest of the group east, winding throughout the path. It's a good idea to put two or three stronger units up front while ranged attackers like Artur and Lute behind them to weaken the upcoming enemies. Be sure to keep you're weaker characters out of range of the second ballista because the archer will take full advantage of the moment they step in range. Use a unit with higher defense like Gilliam or Seth to soak up the Ballista shots to have the rest of the group through safely. If you brought along Colm, he can steal an Energy Ring from the mage in the middle of the map.

Boss: Murray
Use someone with a lance when he's attacking close range and use a hand axe for long range attacks. He can really hurt some of your units if you're not careful and only a few can actually attack and survive. It's highly advisable to NOT attack on your turn and instead wait for him to attack or you put yourself at risk or losing a character. Try to have a chain of rescuers for easy healing. Once defeated, he drops a Knight Crest which you can use on a knight or cavalier.