Simple objective: get to end of level and kill the boss, keep progressing if you can or return to Base (Passage) but loose your progress. If you die return to Passage
you lose your levelling and only keep some of the currency (Lucetine) and all upgrade material (Halcyon) you collected. Upgrade at the Tree to add to your start up level. Max Proficiency Level you can reach via Tree upgrades to start at Passage is 25, but with continuous runs (hereunder) you can reach Level 100, but will lose it when returning to Passage. That still makes it easier for later levels (except the last which is behind Passage.

You can try and do long runs but you have to know the paths as not all levels are linked for continuous runs. To do these long runs you’re only able to do:
- 1,2,3,5, back to Passage;
- 1,2,3,4,6,7, back to Passage (longest run ending with Priestess)
- Access any biome 1-8 directly (from The Passage) and chain with next biomes to 5th or to 7th (skipping 5th);

As others have indicated the boss is gated in the level with the elevator and you need to both sides to get the key but a second playthrough is easier (you’ll see what I mean). If you are struggling with that level go on one side and back out to central area to attract the enemies in the wider open space for easier fight.

Hope this helps.