I kicked his ass on Lunatic by using a team formed by: Ryoma (5 stars, level 40), Bunny Camilla (5 stars, level 39), Maria (5 stars, level 40) and Nowi (raised to 5 stars from 4 stars, level 38) xDDD
Summary of what I did:
1st turn.- I used Camilla as a bait for the Troubadour which in turn made Camus move to the nearest space he could go towards my units and moved my other units to prepare a defense on the left side.
1sr enemy turn.- The enemy Troubadour moved in and attack from two spaces away and Camus moved into the nearest place to my units he could move in this turn. The other units just moved to the top of the map for some reason, I don't quite get :/
2nd turn.- Then in the next turn, I killed Camus by using Camilla and landing the finishing move with Nowi (she got a perfect level up and got to level 39). After doing that, I healed Camilla with Maria and made sure Ryoma was ready to go block the Axe Cavaliers in the next turn.
2nd enemy's turn.- Camilla was attacked again by the Troubadour (I made sure that by attacking Camus, she wasn't in range of the Red Mage) and the Red Mage attacked Nowi, the other remaining units just moved closer
3rd turn.- I just got Nowi away (she was attacked by the Red Mage) and healed her with Maria. Then, since the Troubadour was on the way, I moved Ryoma to do the killing and got rid of him. Then, since Camilla was now in range of the Red Mage, I moved her away.
3rd enemy's turn.- Enemy's Red Mage tries to do something to Ryoma and he regrets it (low health for him and Ryoma almost doesn't take any damage), the Axe Cavelier tries to do the same and he also regrets it (Cavalier is left with 1 health and Ryoma doesn't take any damage), the other Axe Cavalier moves closer to follow his comrades steps in the next turn xDD
4rd turn.- I killed the Axe Cavalier with Camilla, so Ryoma can took care of the Red Mage (only actual menace for my other units since Ryoma was already blocking the way and can one shot them all). I healed Nowi with Maria (just to do something with her) and I also used Nowi's Defense Rally with Ryoma.
4rd enemy's turn.- The last Axe Cavalier commits suicide by attacking Ryoma and the battle ends :P
Hope this useful for anyone having troubles with any of the difficulties of this GHB :)