Petrify has a rather high success rate for Harborym.

Change your Ghost's (a type of unit) magic if you equipped FireBurn for the last mission and don't normally use it.

The enemies will match your own level (which should be 20), while the leader, Irvine, will be level 21.

VanCharm and FolCharm (types of spells or abilities) would serve you well as half the enemies are Earth Element, while the others are Water Element.

The enemy is divided into two groups of four and six. Aim to wipe out the smaller group across the river from afar, where the Black Dragon is the only direct attacker.

Move Canopus straight ahead and throw a stone at the Dragon. Sara should do the same with an arrow.

Aloser should use magic.

Make Byan wait to prevent a Lizard Man from using Cure+ on the Dragon.

Harborym need not move for now.

Your Ghost should move behind Sara and use magic on the Dragon, provided the Lizard Men have had their turns.

Denim and Presance can move closer to their allies.

Have Forcas move next to your Ghost and target the Lizard Man behind the Dragon; this will instead hit the Dragon, finishing it off.

Continue to make Lich Byan wait, and from here, proceed to wipe out the other three characters across the river.

When Canopus' turn comes up, one of the Lizard Men may have moved away to use its Crossbow on him from afar. Have Canopus stay as far away as possible and use Tornado on it rather than be lured toward the Wizards.

The Wizards will most probably separate, one going below and one heading across the 'bridge'.

As the other enemies approach, your warriors will be ready for them. Your indirect attackers can provide support from afar once they have cleaned up the other side of the river.

Harborym can use Heat when he has spare turns.

Make Lich Byan cast Quick on whoever would benefit most.

The leader Irvine (Terror Knight) has a Cure+, which means once you wound him enough he'll use it and retreat, giving you more time to finish off any regular enemy survivors.

Eventually, surround the leader as per usual.

Collect any scattered Stat Cards.

Before you finish him off, ensure everyone is healed as you will go straight to the next battle without being recovered.

In the next mission, you get to choose to fight the leader one-on-one with Denim, or a normal fight against ten foes (including the leader).

Fighting with just Denim is a bad idea because you will lose out on the EXP which would boost the rest of your team to level 21, not to mention any Stat Cards.

If you choose to fight one-on-one, be sure to let Denim deal the killing blow to Irvine so he reaches level 21.