Alright, after dieing twice on hard, I managed to beat lunatic after 3 attempts. Changed my team a bit though.
Team:
- Young Tiki (+HP Seal, which was necessary)
- Cordelia
- Kagero
- Julia
All were 5*, Young Tiki was +1, all level 40. No optimal Skill Inheritance in the slightest.
First, I move Julia within Range of the mounted archer so that only he could attack her. Kagero was below here, Young Tiki to the left, Cordelia to the right (out of range of everything). Julia brought the Archer down to around 22 HP or something. Everything moved and the blue mount moved the archer behind him. Next turn, everything moved so that no one was in attack range of anything. I think Julia moved 2 tiles down, Young Tiki stand where she was, Kagero I think could also remain there and Cordelia move either on or behind the mountain on the left side of the river.
Next turn, Cordelia killed the archer and was pushed over the river by Young Tiki, who inherited the 2 tile push skill. This was also necessary for my tactic to work. Kagero and Julia both attacked Linus and Tiki was blocking the path. She now had to survive both the blue horse and Linus, which wasn't possible without the +HP seal for mew (42 HP and 42 damage taken without the seal). Linus gets counterkilled and the horse swaps position with Tiki, who had 3 HP left.
Next turn, Kagero and Julia (and if necessary, also Tiki) kill the blue mounted unit. Cordelia kills the blue mage and escapes to the south after Galeforce activates. Now you only have to retreat to the bottom left corner and lure the axe flier into range of your units, so you can kill him.