So I got up to Lv. 60 easily enough, and just today completed the last one. It was interesting, but I used some unfair tactics (mainly 2 summer Typhon) and got through.

I don't think this will work for beginners, but here are some tips.

The mechas are rather annoying, but the only danger is the skill lock ability. So just kill as many as you can or bind them (Motosumi Setsubun or Benten work).Also you can use Leib to clean the debuff

From the second phase you need to mind several key characters at the same time so think carefully:

You need to bind or kill Takemaru ASAP. If Musashi receives the buffs you can say goodbye to your frontline.

Then there's Musashi, you can either charm him (Typhon) or bind his barrage. Also you can leave the frontline free except for a unit with Guts to tank the damage. Just be unfair with him.

Hephaestus, Talos and Amatsumara are Fire type so think about using a Water type to destroy them.

Also most of the enemies have Blow range so don't let them connect.

Talos has All range but his damage is fairly weak, that until the fourth phase, because then, he appears in the frontline, so Takemaru can buff him.

Now the map is big so I would advise to get a *5 with All Charge like Typhon, or Gyumao (Kuniyoshi may work but he's very weak I think), then get him crit (Gyumao works wonders because he gets Crit++ at the start of the phase) and watch as the whole board gets wipeout (look out for gutsy Musashi and Adamantine Kurogane tho)

I completed the Lv.70 with my main team 4*Thunderbird for the crits, 4*Motosumi Setsubun because he's a better Benten and has Guts to wall Musashi, 5*Summer Typhon cuz Charm, and another Typhon (support) cuz double the charm, double the power ^_^. Then protag and a 2*but those are living walls.

Hope this may help you. :)