Do multiple really short runs. Each run should take under 2 minutes and you should get from lvl 1 to lvl 9 in about 12 runs.
- choose highest difficulty you have unlocked
- activate custom mode
- set negative modifiers: Berserker Foes, Angry Birds, Blood Lust, Disease and Inflation/Dried-up Wells (do not pick Oppressive Nightmare or Shadowy Fog)
- start the run
- immediately head towards the boss
- defeat some minor enemies on the way to gain xp
- start the boss fight by standing inside the boss area once you're there
- once the boss fight started, abandon the run using the menu
More details:
https://steamcommunity.com/sharedfiles/filedetails/?id=3451333701