Levels come from kills, quests and crafting. Quests are the fastest and safest, which makes them the obvious core of a levelling plan.
Quests first
Trader quests give XP, dukes and loot in one trip. Stacking several in the same POI doubles the reward for the same walking.
Horde night is XP
A blood moon you survive is one of the largest XP events in a week. Build to survive it, and the levels follow.
Watch game stage
Levelling raises your game stage, which raises horde difficulty. Fast levelling without better gear makes night seven worse, not easier.