I am up to 3-7 and I agree with the difficulty spike. I run a mixed team with a 3* Soldier, 3* Knight, 4* Healer, and 3* Archer. I generally bring a melee character, either knight or soldier for the guest slot, and have her strafe up and down in the back row shooting down ranged targets or adding her damage to a character that has more than one enemy on her and it works really well for me. I have little to no trouble with quests at this point up to about level 17 average. I pushed hard on the weapons sale deal to get all my characters up to their B weapons and upgraded as much as I could, which made a huge difference as well. On some of the lower level missions my soldier and knight are only taking 1 damage for most attacks. One of my strategies is to use my soldiers dash ability to get behind targets that have ranged attackers behind them so I can take those out faster or in the case of a boss, get another character in front of him so I have 3 attacking him at once, then spam the knights/archers abilities and use the healers for healing when her regular heal can't keep up.