Keep completing the floors available when reaching the higher levels
The general strategy of training when reaching the higher levels is to keep completing the floors available to you and resetting your progress when all unexplored dungeons have been completed. This will increase your Previous progress number every time you decide to reset.
Tip: If you want to complete the lower-level floors quickly in order to gain progress faster, set your complexity level to 1. This lets you complete floors faster since you will be provided with equipment at the start, and the dungeons will be smaller. A disadvantage is that the experience you get is heavily reduced.
As you get to higher levels and floors, you will realise that gaining access to new floors will become more difficult. Activating your Prestige experience build-up at this point will be a good decision to maximise experience earned. As with before, it is crucial to set your complexity level to 6 when completing the highest level floor that you can do to minimise the penalty in experience.
There are a number of different ways of completing dungeons for experience at this point: Solo rush - rush the dungeons by yourself either on complexity level 1 or 6, discovering every room and leaving out as many monsters as possible. Only recommended if you only need to complete one or two floors in a row. Setting dungeons on complexity 1 offers the advantage of completing floors in minimal time, while complexity 6 lets you rush floors while gaining large amounts of experience in a Prestige run
Duo team rush - gather a group of people to quickly complete the lower floors on either complexity level 1 or 6, leaving any dead-ends alone, unlocking every room and reaching the boss in as little time as possible. Each of these dungeons should only take 10 minutes at most
4 or 5-man large dungeons - preferably done on higher floors, large dungeons offer the most efficient experience by far. Leave dead-ends if the monsters in there are too strong, but kill as many as possible. Try to unlock as many rooms as you can. It is very important that you do not die in these dungeons as you may lose a lot of potential experience!
Because the experience you get for completing floors is accelerated, it will not take very long before you reach the average levels of 40 to 50, and perhaps to the higher levels of 60+. As with before, discover any new bonus dungeons that you gain access to as soon as possible to maximise experience.