This is easily done with Mighty Oak, the first boss. Get Anna, Matt, and Natalie (Lance isn't in the party yet) to a decent level, should be no problem at 11 or 12 (you'll need to turn off "No Grinding" to actually grind, remember to enable 2x fight speed for it to go quickly). Mighty Oak's weaknesses with "More Foe Resistance" include Fire and Thunder, so Anna should come with Spark Arrow lv. 2, Matt with Fume lv. 2, and Natalie with Fireball lv. 2, also give all available Ham to Matt and Cake to Natalie beforehand for higher damage. During the fight, buff Matt with Temper and Natalie with Charm (you can buff Anna with Temper as well, although I mainly used her to keep the party alive with Renew and boost the party's defense with Protect). Make sure all characters enter with full limit breaks, so Matt can deal high damage with Cleaver and Natalie can heal/buff the party with Kyun if needed (Anna's Mother Earth won't be useful here). Ignore the weaker enemies spawned by the boss and just focus on beating it quickly.