Just completed the "Fatal Error" match in the arena on my second try. That was a pain the ass. I think those Geth Primes were harder to take out than the other enemies in that last wave.
The Banshee went down quickly and then I left Grunt and Wrex to soak up the hits and damage while I retreated to another corner. Then I took out the Atlas, Primes, and Praetorians from afar while they were focused on trying to kill the Krogans.
Also, I love that the "Shifty Cow" appears on the scoreboard after getting over 9999 points.