Finally did it, with the help of the cheat menu to circumvent an annoying bug.
We defeated Erebos in the evening of Day 23, finishing with a level 35 party (level 32 when the Battle of Karthal began).
Honorine: Paladin; G Light, Sword; M Heavy Armour, Shield; E Crossbow, Warfare
Kimaira: Elder Druid; G Water, Earth; M Prime, Magical Focus, E Bow, Medium Armour
Torian: Pathfinder; G Axe, Crossbow, M Fire, Medium Armour, Dual Wield; E -
Lisa: Marauder; G Spear, Dodge; M Air, Bow; E Two-handed, Arcane Discipline
All quests done (incuding Shrines and Obelisks; Forbidden Saga complete), all enemies killed, all treasure claimed.