Found a nice combination for my Ironman mage. It's really effective and save.
Use sunken citadel and ark, you can have an endless run without risking too much and not losing 12h for an inn. It's less effective than icemist underlevels or ark (main floor and reactor room), but it's way more saver.
SC needs 12h for reset, Ark 6h, so clear SC, don't forget to remember the ingame time, go to ark, clear main floor, use medical Station and go back to SC. That way you can use XP scrolls without losing time.
Here a video how it works
Ps: it should take about 6h for 1 million XP. And you won't risk too much with Ironman.