In those runs 1-turned Zeth on turn 5.
I think my run was pretty standard and nowhere near perfect:
Turn 1 - Defend Rance Castle, Evacuate Pencilcow
Turn 2 - Defend Free Cities, Leazas Campaign 1 (defeat La Hawzel)
Turn 3 - Defend Zeth, Helman Campaign 1 (defeat Babolat)
Turn 4 - Defend Zeth, Free Cities Campaign 1 (defeat Lei)
Turn 5 - Defend Kawanakajima, Zeth Campaign 1 (defeat Galtia + Medusa) - all that work on Zeth and they still couldn't self-liberate by turn 13.
Turn 6 - Free Kamilla, Find Miki - wasted a prep phase here rescuing Kamilla
Turn 7 - Free day (3 chests), Shangri-la Campaign - one of the chests had Krutche in it that I wanted for Hornet rescue, but likely cost me Land ending
Turn 8 - Defend Rance Castle, Save Hornet
Turn 9 - Defend everywhere, Helman Campaign 2 (defeat Kesselring)
I originally did Leazas Campaign 2 on 10, Free Cities Campaign 2 on 11 and liberate Helman on 12, but I couldn't get an "A" ending since I messed up on the boat choices (no "sea"), didn't have the "sky" quests appear, and could not get both Free Cities and Leazas to self-liberate by end of turn 13 for "land invasion" (Free Cities self-liberated end of turn 13 but BOTH Leazas and Zeth needed to end of turn 14 to self-liberate). I went back to end of turn 8, fixed the "boat" choice, and then free-dayed to turn 12 to take the "sea" route (had to defend Helman and Free Cities once each of else they'd have been annihilated).
After winning on "sea" route, I could now get the events for "sky" route, so I went back to end of turn 9 and did -
Turn 10 - Reinforce Leazas and Free Cities, Poppins Tunnel (defeat Masuzoe)
Turn 11 - Sky Route prep, Fight Troppos
Turn 12 - Sky Route, Sky Route
Turn 13 - Sky Route, Sky Route
Turn 14 - Sky Route, Kayblis Castle
If I went back and re-played from the beginning (with 0 CP), with the aim of winning by "Land Invasion," I would do it like this instead -
Turn 1 - Defend Rance Castle, Evacuate Pencilcow
Turn 2 - Defend Leazas, Free Cities Campaign 1 (defeat Lei)
Turn 3 - Defend Free Cities, Helman Campaign 1 (defeat Babolat + Kesselring)
Turn 4 - Defend Helman, Find Miki
Turn 5 - Defend Kawanakajima, Zeth Campaign 1 (defeat Galtia + Medusa)
Turn 6 - Reinforce Zeth twice, Shangri-la Campaign
Turn 7 - Defend everywhere, Leazas Campaign 1 (defeat Silky)
Turn 8 - Defend Rance Castle, Save Hornet
Probably on a clean re-install so that you don't need to deal with Warg. One-turning Helman on turn 3 is difficult but feasible. Using Lei, I had no trouble toasting Babolat in 5 turns and could deal ~ 40% damage to Kesselring before he turned mist. With some luck, all you need to beat Kesselring is to survive ~18 rounds, and zap him for 50% damage during each of the two Fog phases.