My sub-bases is leveled (with a 2* slope from the high long side to the low long side, it’s cut into a slope) red clay, wet and vibrating rolled/tamped, with screened sandrock as the base. I don’t remember the original depth but it was vibrating rolled/tamped to a final 6" or so. THEN I have my riding surface on top.
this lets the water drain through the soft footing, through the hard base, and down to the sub-base where it exist downhill.
What is your underlying soil? For mine the height difference between the uphill long and short side meant that material was moved downhill to “level” things. That’s what you want to do IF your natural soil is conducive to a hard as concrete sub-base. That means no new material, just a redistribution.
If you want to just raise the low end, then you still want something that you can vibrate and tamp/roll to be like concrete. If that soil is airy, then you will need to do some combination of geotextile fabric and a crushed rock base (screened sandrock, screenings, bluestone, etc)
Find out who has done local arenas that you know have held up, and see if you can talk to them
The guy who did mine did road grading as his profession, so he new all about what it means to grade and compact
