Some basic tips to help you beat the story chapters. Recommended levels are displayed in brackets.

AOE = Area of Effect, ST = Single Target, DoT = Damage over Time.

Chapter 1: Ba’al (4)
Just level your characters and you should automatically win.
Psychological Flames (ST) every 3 turns, Counter Flame (AOE) when below 25% HP.

Chapter 2: Leona (6)
Guard when she uses Holy Ward.
Holy Ward (Shield) every 5 turns.

Chapter 3: Iblis (10)
Guard when she uses Iblis’ Magic Ceremony/ Protective Screen and before she uses Poison Rose (turn 6, delayed by 2 turns if she uses Iblis’ Magic Ceremony). That’s it.
Poison Rose (ST) every 5 turns, Iblis’ Magic Ceremony (delayed AOE) at 75% HP, Protective Screen (delayed AOE) at 25% HP.

Chapter 4: Fiora (14)
Make sure you have an AOE attacker like Iris. If you’ve cleared all the free quests in every map up to this point and your units are the same level as your account, you should be fine.

Chapter 5: Satan (20)
Don’t attack her when she uses her ability (Brutal Brutality), guard instead. She will heal herself once during the fight. If you still can’t beat her, level up a bit.

Chapter 6: Irene (28)
Don’t kill Irene first, or you will get instantly wiped. Try to aim for Kani (dwarf), then Petra (mage), then get rid of the other enemy attackers. Irene should be the last target you kill.

Chapter 7: Lana (35)
She uses her massive AOE ability every 4 turns, as well as an instant kill version when she’s below 475,000 HP. Try to burst her down and bring damage mitigation (whether in the form of Obstructors or Shielders).
Earthquake Slash (AOE) every 4 turns, Dive & Destroy (ST) every 6 turns. Dive & Destroy will take precedence if they coincide.

Chapter 8: Lulu (40)
Very easy once you figure out the gimmick – take out the two guards on the same turn. Do not kill Lulu or kill either of the guards on their own without taking out the other, or you will instantly lose.

Chapter 9: Salucia (47)
She uses Guided Arrow at 75% HP, Archery – Skybreaker at 50% HP, and Leave the Battlefield at 25% HP. You should be guarding all of these attacks, though Archery – Skybreaker is the only one that instantly kills your team.

Chapter 10: Shizuka (52)
Don’t attack her with the units that get debuffed by Explosive Edict (look at the words on the screen), or they will instantly die. Make sure to check the debuffs on your characters with the button next to the AUTO toggle.

Chapter 11: KS-VIII (55)
Guard the first turn, then guard her other Ultimate Ability casts if you have to. She gains a large shield at low HP. Survive for a bit and you’ll automatically win after that phase.

Chapter 12: Juneau (60)
Take out Juneau last, as she enrages the entire team below 50% HP. Kill the rest one at a time, because they self-destruct with a powerful AOE. All of them enrage individually below 50% HP. It’s recommended to take out Natasha first, followed by Prototype #3, followed by Blaire and then Anna.

Chapter 13: Minayomi (60)
Guard when she uses Iaido Stance and hit her twice when she uses Power Up Stance. Make sure to finish her off quickly after Still Water, as she gets a massive attack buff.

Chapter 14: Ritsuki (60)
Bring enough healing as she applies AOE damage over time. She will debuff specific units with Flaming Toad, which you will have to guard (or else it applies a massive DoT on that unit). The same goes for Slimy Slug, which silences the unit if you don’t guard. She leaves the battlefield at 25% HP.

Chapter 15: Archangel Saint Milae (60)
This is a fight that gives you a few ways to play.
AOE Team:
The 2nd easiest is bringing an AOE comp like Ritsuki + Britney + Iris. The fight starts off with a 50 turn DoT, which will have more the longer the fight goes. The other angels give her various buffs, but with an AOE comp, they are not particularly important. After registering the death of an angel, once they get below 25%, on turn 4, the angels will use 50% of their health to heal ASM, which will generally bring her to full health and she will get an initial buff that makes her take 33% less damage each attack each turn for the rest of the fight. This means that after 3 hits, she will no longer take damage. The following turn, she will go into Guard stance as a charge up to an AOE attack which you can guard against, if needed. It pierces shields as well, so don’t get complacent. This AOE makes her 100% susceptible to CC effects, and if you have them available, go nuts. If you don’t, she’ll cast her next AOE DoT that lasts for the rest of the fight. Once she gets below 25% health, she will cast Unshakeable Faith, which prevents her from taking damage for 3 turns (300% damage reduction) which also affects DoTs placed on her. Rinse and repeat for the AOE Comp.
Single Target Team:
For the single target comp, you want to focus on one Angel at a time, since each Angel gets a buff from ASM to increase the enemy units’ Attack and Ultimate skill power. Once an Angel dies or get’s below , ASM gets healed and a buff to take 33% less damage each attack each turn for the rest of the fight. Heal and guard through taunts as needed. If you are unable to take out an Angel in one go, she will heal ASM and give her a buff, so try to take them out in one go. All units are susceptible to CC and CD reductions, so a 5* Manuela or a Britney can massage this fight and make it easier.
Recovery Debuff Team:
This is the easiest way to play. You can focus on ASM and once she gets below 50% health, the other Angels will sacrifice half their health to try and heal her. This ends up with all the enemies at 50% quickly. However the cost of this is that she gets all the buffs, 2.5% HP healing each turn (ignorable), increased attack power, extra damage on all your girls for the rest of the fight. If you are able to kill ASM or reduce the other Angels’ HP below 25% with the Recovery Debuff at 100% on ASM, the Angels will die and apply their buff a second time to ASM, so bringing an AOE unit makes this a walk in the park.

Chapter 16: Genius Tactician Britney (60)
This is a pretty straight forward fight. All units operate in much the same way as their normal counterparts except that Britney is the unity you need to kill. AOE Units help to deal damage to her during taunting turns. Once she gets below 20% health, the enemy units guard while she charges up for a multi-hit attack that partially ignores shields. From there the fight resumes as normal. Once she is gone, the remaining units in the team will leave the battlefield the following turn.

Chapter 17: Ancient Warrior Uruta (60)
Interesting and slightly random fight. AWU has CC Immunity, but she is affected by CD Reductions. She also removes “99” debuffs each turn, so DoTs and Defense Down units are not as viable. A 5* Manuela makes this fight easier as she won’t use her Ultimate as often. She hits pretty hard 2x a turn on single targets and every turn gets an Attack buff. She’ll cast a medium hitting attack called Magic Bullet every so often starting on Turn 3, which gives her a Counter Attack buff for 1 turn at 400% her attack. This can be ignored once or twice, but will eventually one shot you if you forget to read. This is usually followed up by her Magic Blocker Ultimate, which does heavy damage that partially ignores shields, then gives her a massive shield and makes her all but immune to damage (500% less) for one turn, so heal and guard as necessary. Starting on Turn 6, she will say that she needs to pee followed by 2 buffs assigned to one or two of your units. The buff is for 1 Turn and requires that unit to use their Ultimate, so save it if you don’t have a 5* Manuela. One buff Increases AWU’s Ultimate skill CD by 1 and the other Damages AWU equal to her current shield. There seems to be a glitch with the bars and sometimes they don’t update, but you should use the Ultimate of the unit that removes her shield first, then attack with your other units. She will then perform an AOE on your units. From there it is a rinse and repeat race to the finish.