I made it to Rank 18 on my first foray into the Colosseum, but none of my 9 units could withstand that archer/cavalry assault. I don't want to fuss with moving people around to get through it yet. I think it's a pretty good start to get into the lower 20s with my unoptimized 14-16s.
...it would be wrong to sit and grind through the auxiliary battles for hours to get the +2 AP/PP accessories... Yes, wrong...